Entirely subjective, and limited to those I have employed in PVP. Also, I don't have the entire arsenal available for my agent, so there are combinations that I could not try, again affecting the outcome. For convenience sake, unless I own two extra outfits, I have only reviewed the best outfit I have unless I have changed classes.
P5 Emma Frost, Tactician
I'd have to say, she's by far the best PVP hero to date in terms of both AFK and on use, versatility, and survival. Her only limitation lies in the damage she deals out, but even that's only a minor quibble if you put the right crystals in and use her Mental trauma. With the abundance of Scrappers, especially Ghost Rider, I doubt the Infiltrator setup is as useful. Also, against Mockingbird, being a Tactician as opposed to being an Infiltrator could spell the difference between survival and one-turn death.
Overview of her skills:
1. Psychic Tap: Its direct damage is very low, although if it crits it can deal 700+ points on its own. Its true damage comes when the debuffed opponent attacks, at which time the Mental Anguish can deal up to 5400 pts, or nearly half his total HP. It's a great move that discourages attacks from the player (not so much the AI) and the fact that it's psychic means that it bypasses most shields, effects, and cannot be countered. It still triggers class-related effects, but even that could be used to lure others to attack with the Anguish on.
2. War Diamond: It deals nice damage, but is a very plain melee attack that hits twice and has a chance to stun. Since its stun rate is quite high(60%), it's a nice way to stagger the opponents build-up plan and also useful to setup other teammates that cannot get through protection (if it stuns Cap/Colossus).It used to get rid of debuffs, which made this incredibly overpowered, but even at this stage it is nice to have. My use of this is mostly to finish the opponent off when they have only one standing, or, like I said, to prevent strategic buildups. Stunning the Agent, for example, could delay the use of their Quantum Jumper or Coulson's Revenge.
3. Unlock Potential: Best use of this is against another Emma. You want to use this to dispel her Mental Anguish on ANOTHER hero. If it's your Emma that she placed MAnguish, it's best to ride it out and UP your other teammate who will deal damage instead. Mostly nowadays I use this to get rid of debuffs (especially sin stacks).
4. Mental Trauma: Amazingly powerful considering that it does not trigger anything and cannot be blocked by shields or digital decoys. With this, Emma rounds up her arsenal with a potential weapon to use in all situations - a powerful damage-dealing attack against non-protected, non-countering team in War Diamond, a single attack that potentially sets the opponent up for big damage in Psychic Tap, and an AOE attack that bypasses digital decoy(something Psychic Tap doesn't get through). Not only that, it also places Migraine, which seriously decreases damage dealt by the opponent, such that this could just as well be the main attacking skill if not facing a group of infiltrators.
Overall, Emma's value is in her strategic use and making the right decision when using her is extremely important. Although normally this means that she's really not that great for AFK PVP, her P5 special pretty much negates this and her skills are fairly poweful enough regardless. But beware that the AI will misuse Unlock Potential such that it basically wastes her turn. Fortunately, the AI can only do that once every two turns even if it tried to screw you up.
USAGE: Like everyone else, her usage will depend on the team faced. First I tend to check for debilitating debuffs. Mental Anguish on Emma or to my hero/agent who doesn't have a way out will be considered first. Then, if there are no tankers on the other team, War Diamond on a non-countering opponent is considered as the stun rate is fairly high. Normally I'll take out the agent with the Quantum Jumper, but this will depend on whether the other team has Emma. Cosmic Power cancels stuns, so unless I want the damage done, I end up using Mental Trauma. Mental Trauma is also used against teams with high firepower to get Migraine applied.
AGAINST: There are two types of Emmas, one that MT's a lot and another that PT's alot. The former really can't be prepared against but at least it does not need much know-how, either. PT'ing Emma's can be dealt with using Unlock Potential on targets other than Emma. Know that if you have Mental Anguish on a hero/agent with a defensive skill, it's better to use that skill to get Mental Scarring rather than removing it with Unlock Potential unless you really need the damage for sure.
Ghost Rider, Scrapper
I've just recently come on board on Ghost Rider and he's been quite useful even at level 10. What he offers is just an incredible amount of debuffs (sin) and a great psychic attack in Penance Stare that allows him to deal quite a bit of damage against Bruisers. Basically he works as your insurance against Quantum Jumping turn devils, and his Infernal Contract does actually work at times to save your life. His main problem is that he has nothing initially against teams with passive tanks, but it's up to you to design a team to shore up that part of his game.
1. Damnation Chain: I rarely end up using this skill unless the other skills are on cooldown. It's got nice debuffs going for it but with the load of P5 Emmas, it's difficult to work its way up. Burn-Dark Void-Soulfire is very nice, and since it doesn't necessarily have to be from the chains, it works well with Dark Sigil or even Vapors of Valtorr. You could possibly create a combination that takes advantage of this using Dark Sigil, Strange and some Spec Op gear, but I don't have them so... By the way, it's Melee, which means it will not deal 100% against Inf. Agents with the PVP suit. It's also Fire, so it won't work against the Human Torch.
2. Penance Stare: This is one overpowered skill that you really have to get the feel for. It does not do the initially estimated yellow bar damage; it deals quite a bit more due to its high crit. As it ignores defense, it's best placed against bruisers that GR normally shouldn't have an answer to. With Sin stacks and Vengeance stacks, it really works nicely but you have to time it right against P5 Emma teams, since Cosmic Power and Unlock Potential will get rid of Sin stacks. Worth its use every time, just about. As a psychic attack, it will go through MFG and bypass tanking. Oh, and since this is Psychic, it cannot be countered or protected against, and more importantly, if you have Scrapper bonus, the second attack(DChain) will not be countered or protected against either.
3. Highway to Hell: My favorite opening move, especially if the opponent has an infiltrator. Melt armor means that you can apply this to bruisers and only worry about taking more damage, and it applies soulfire right away to the other two targets - again, if you have that Spec Op weapon that takes advantage of Soulfire, it may worth a look. It also hits 4 times, meaning that it will eat through digital decoy.
4. Burnout: Quick action, and it serves as a quick buff for Ghost Rider. Great except that it can provoke counterattacks all across AND enrage bruisers, so must be used wisely. It deals quite a bit of damage on its own when the opponent has a few stacks of sin decreasing their defense, and even without any sin stacks it increases damage a bit.
He is also immune to burning, which is nice, and has a chance to live through a fatal attack. All of this can be very handy at times, but hard to rely on.
Overall, Ghost Rider has skills to bypass most sticky situations other than Mental Anguish, at which time you'll have to judge whether to rest it out or to proceed with it. Penance stare is truly powerful and can one-shot enemies, so if playing with Emma, it's worth setting him up with Unlock Potential to maximize damage. Remember that it's Psychic and barring the Experiment(digital decoy too, likely), you can attack anyone you please, go through any shield there is and just take one out. He also punishes every team without Emma as he has too many debuffs going. USAGE: Highway to Hell is usually the best opening move, except when the opponent went through a bunch of turns and got Sin stacks all over. Then it may be worth opening with Burnout. Always remember that other than Penance Stare, nothing goes past protection and that everything will trigger counterattacks. I personally find Emma to be a great target for Highway to Hell since it somewhat negates her Diamond Body, but Soulfire is a nice debuff so it depends on whom you want Soulfire placed. Generally Damnation Chains aren't worth it because the fights rarely last that long and because the other skills are much better. However, against a weak Infiltrator in a team without Emma, I could see Damnation Chain being worth the use. Penance Stare will be the main move - just remember that with fights against Emma, one or two stacks of Sin will probably be the most you'll get to see.
AGAINST: Got to remember that GR is immune to burning. The AI screws up a lot, especially if it has the Human Torch, by trying to apply its lvl 1 attack against him. Don't make the same mistake. Faulteaters and such are thus better placed elsewhere. Getting rid of Sin stacks is also something to keep in mind - three or more you're probably going to get one-shotted by him. Otherwise, pretty easy to deal with as GR doesn't have much in terms of defense or counterattacks. Mental Anguish works exceptionally well (has no natural skills to wear it out) as does Mental Trauma since Migraine decreases his damage quite a bit.
FF Invisible Woman, Infiltrator
If I had Mockingbird, she'd probably come here due to her excellence in AFK fights, but since I don't, it's IW. She is just as versatile as Emma, with the only exception being that she doesn't have Psychic attacks that penetrate shields. However her Force Cage is a mean bugger when employed correctly, and although the number of Scrappers make her employment risky, the number of Tacticians make it worth risking, especially when you're initiating combat.
1. Force Spheres: Very decent damage with Deadly Crits. Stealthy, so you don't have to worry about tanks getting in the way. Gets stopped by most shields, but the damage it deals makes it a great mainstay.
2. Force Field: In PVP, it is such a huge shield that it can absorb a round of Mockingbird most of the time. It's also the first move she'll make when she has Infiltrator Counter Attack, so plan accordingly when facing Tacticians. It's useless against Psychic and other shield ignoring attacks, namely Coulson's Revenge, but does wonders against Captain America and Colossus. Also, it's a very nice way to get rid of Mental Anguish. But always try to guess what the opposing teams attacks are. Against a Psychic team, this is a turn wasted.
3. Force Volley: Basically a spread-out version of Force Spheres. Shares the same faults as well, except that this one gets through Digital Decoy. Does decent damage for an area attack but is probably the worst thing you can use against a team with Scrappers or a team shielded by MFG. Often it will take off all three MFGs at the same time, and the result isn't always pretty.
4. Foce Cage: Widowmaker, Pinpoint Target, Critical Intel, and War and Peace can be applied on targets with Force Cage, but other than that, the target is nearly completely shielded from damage/effects. It's a good thing to place on tanks to stop them from protecting/countering so you get yet another way of bypassing tanks. Unfortunately it is removed by Cosmic Power, and Scarlet Witch could make mockery of this with her Lvl 6/9 skills. Still, if the opposing team does not employ Emma, you could practically eliminate one of them from combat. Although that would make it 2 on 2 with perhaps the opposition using defensive skills to bolster his/her team, it could be worth it depending on the match up and even better if IW can passively participate in combat through Coordinated Attack, Combat Reflexes and what not. Honestly, however, most of the time she deals enough damage to just outwork most opponents without the use of this skill.
She also has a couple passives that is worth noting, especially her FF shield. Fourth Field gets applied after her first turn, and it's actually big enough to stop most area attacks that aren't psychic. Her invisibility that automatically dodges attacks does not get triggered often enough and the bonus to Accuracy is somewhat moot, but I find higher Accuracy tends to lead to higher chance of crits, and with her attacks all being deadly criticals, it helps.
Overall, she and Emma make up my white angel team, a team that simply tries to win out using the Agent's firepower and making the opposing team disfunctional. Lately, however, with too many Ghost Riders, having an Infiltrator has become too risky - especially since Mockingbird getting Scrapper bonus on her means that MB will nearly kill off two of my team members in a short time.
USAGE: When faced with Tacticians, you have the choice of either hitting them with the Force Spheres to get the infiltrator bonus, or Force Fielding your own team. If you are facing a team with MB, you definitely want to shield yourself first, but otherwise I'd go for the bonus especially when facing Caps or Colossus (they hit with area attacks first almost half the time, so you'll get the shield; also the Fourth Field given by FF suit usually is good enough). If the opposing team has Scrappers, don't use Force Volley unless their Scrappers already have the bonus. Force Cage is something to consider if the opposing team cannot wear it off. The Signpost is wicked in that it will penetrate the Cage, so if your agent has the Signpost and you see a 2:1 situation, you could eliminate all doubt and simply Force Cage - Signpost the opponent to death.
AGAINST: Honestly, I rarely come across her, and she's not that much a threat if you have a team with Psychic attacks. None of her defensive skills can deal with Psychic attacks, so Emma can actually deal with her just fine by using Mental Trauma every time. Also the Infiltrator version just gets rocked by Scrappers, especially GR who will apply a ton of debuffs while at it. But since she does pack a punch, especially against Tacticians whom she'll get additional accuracy bonus, it's best to try to eliminate her quickly. Remember that her FF bonus will no longer affect the team if she's gone.