This post refers solely to PVP.
I don't profess to having any absolute knowledge of current algorithms and do not have a full spectrum of experience since I am only level 135; my views come purely from daily playing experience.
I am a working adult, so I am funded. However, even if you are not willing to spend any money on the game, I am sure some of these will still apply.
In addition, I play games to win and aim to reach the top of the pile. I enjoy this game because of the theme and the characters that I grew up with, but I have no emotional attachments - I'll pick an ugly, obnoxious and disliked hero every single time if they are broken.
I have had to break up my views for easier reading, but it is not practical to keep referencing them across the board - it would be best to read the whole thing and put it together to get a good picture of my overarching idea.
I used the same team for both offense and defense in PVP season 13: Blue Suit Generalist Quicksilver, Modern Armor Blaster Thor, and Scrapper's Empowered Suit agent with Hard Nox, Scroll of Angolob, Signpost and Hotshot. My gameplan is obvious - kill the game off as soon as possible, because I do not have a Synthetic Cube. This provides you with the context behind my views.
And lastly, wake up and smell the coffee - this is a pay-to-win game. Accept the fact that someone who pays will beat you hands down because of the raw difference in power - just let it go. Don't get yourself all hot and flustered because your opponent has spent insane amounts of money to one-shot you. That is why they spent the money anyway. Laugh, and move on.
Infiltrators are at an overwhelming disadvantage.
Although the Reflexive Empowered Iso-8 greatly reduces the amount of damage of follow-up attacks by scrappers on infiltrators, their class disadvantage against scrappers is far worse than their advantage against tacticians. After the bruiser advantage against scrapper nerf, I'm not THAT concerned about my scrapper enraging my bruiser opponents, because I am confident of flushing them out with a blaster if I am on offense, and can plan my sequence of attacks.
Moreover, most infiltrators have very low attack, and require set-up to reach their full combat potential. In a metagame that aims to 1/2/3 shot a single character PLUS their disadvantage against scrappers, infiltrators are not viable in the current environment.
Only Psylocke can cut it, with Brown-and-Tan Wolverine or Spiral a distant second and third. Even then, I don't use them at all because I have gotten punished by Quicksilvers in scrapper uniforms.
Don't depend on buffs and debuffs.
With almost every agent packing either a Mystic, a Scroll of Angolob or a Synthetic Cube, relying on buffs and debuffs to bring yourself to scratch or activate certain abilities is not something you want to do. I am not a fan of any hero who cannot do anything if a certain buff or debuff is applied because of the overwhelming number of opponents with Mystic.
I always go for heroes whose abilities cannot be dispelled, or the AI consistently seeks to reapply even if it has been dispelled. I believe these are the only heroes I would ever use in PVP for now:
A permanent fixture in my team whom every respectable player has already mentioned. I don't think I have to say much here, except the fact that I use him as a generalist because of frequency of bruisers in my metagame; this might change if the results of my experiment with Horseman Rogue are good, and I can give Rogue the task of providing Morale Boost.
Guaranteed second/third turn OHKO. If your Juggernaut can't OHKO someone with I'm The Juggernaut, you were not meant to win that fight anyway. AND he's immune to stun. Seriously, wtf? I haven't even mentioned his immunity to psychic attacks.
Single-target protect that cannot be dispelled, immune to all damage-over-time effects, attack and defense increase as HP decreases. He has tank written all over him. I have not met anyone who packs cornered just to disable protect in my metagame, so he is quite a safe choice in my book, although I am not a fan of defensive heroes as you will see below.
P5 Emma Frost.
Built-in damage mitigation, quick action that cleans debuffs and raises stats, plus debilitating offensive moves all round. The only hero that she can't do anything to is Juggernaut, so it depends on how often he turns up.
Rogue (now with Horseman uniform)
Ridiculous damage output that becomes near unstoppable if you do not dispel In The Fray. I always target her first if I'm fighting against her, unless there's Juggernaut in that team as well. With the horseman uniform...I think she's just become ridiculously OP.
Modern Armor Thor (Blaster) It doesn't really matter what the AI does on defense, because I believe it is scripted to use either Hammer Throw or Summon Thunder if there are stacks of Might of Mjolnir on Thor; they can't be removed by Scroll of Angolob. OHKOs Juggernauts and Hulks for me most of the time or at least brings them down for my next agent or hero to kill, and that's all that matters.
Farm your Scroll of Angolob.
The overwhelming majority of my problems have gone away simply by having this stupid item. Mystic Shrouds are no longer a problem for me, as are heroes like Rescue and WW2 Captain America, and Magneto to a certain extent.
Your mileage may vary depending on how your party is set up to take full advantage of it; my setup is designed to exploit it on the turn I use my Scroll of Angolob.
Offense is the best form of defense.
I stopped short of saying "All-out-offense is the best form for defense", because this is not simply a matter of stats. I have never been a fan of defensive heroes because of the sheer number of options available to nullify them and their buffs, i.e. Scroll of Angolob. Moreover, it is far too easy for the opponent to screw up any over-elaborate plan to mitigate what your opponent can do, not to mention that the AI is dumb.
I used to run Omega Sentinel and Quicksilver, because I thought of simply overwhelming the opponent with a ridiculous number of turns per round and the absurd amount of utility Omega Sentinel gave me. However, Mystic put an end to Omega Sentinel's efficacy quickly, and I realised there was simply no way you could recover from a double Summon Thunder coupled with a Juggernaut waiting to keel you over. Even if you survived all those, there would be a Probability Field waiting for you in the third round followed by an Atom Smasher.
This is the reason for my setup in PVP Season 13, and I am sticking with it. My final setup will depend on how popular the Voodoo Staff becomes, which will determine whether I will continue to use Thor. On the other hand, if I had a Synthetic Cube, I am very sure my defensive setup would be different.
Attack and HP are the most important stats.
My view might yet change, but I have come to the conclusion that nothing else matters. When you are deep inside the adamantium league, your aim is to blow 1 of your opponent's characters away ASAP, because they will blow you away if you don't. I used to make my hero stats 5-bars across the board, but I quickly realised something was wrong when I lost 9 out of 10 defensive battles. There is a need to specialise, and Potkettleblack has addressed this in great detail - I shall not talk more about it. In most, if not all games, HP is the absolute stat that increases your survivability in a linear fashion, the most reliable stat to increase defensively once you hit diminishing returns in other defensive stats. Once you hit a certain amount of Accuracy, you only need to increase your Attack with whatever you can, even if the returns diminish.
This is why I've converted all my scrapper and infiltrator heroes to everything else if possible, starting with blasters, then tacticians, then bruisers.
This is in contrast to how PKB builds all his characters - PKB is of the view that evasion is important to a larger extent, while I believe there is no point in increasing it. This is from my experience of never failing to connect, and never being able to dodge any attack regardless of how high my evasion was because I was getting killed by Hulk Smashes, I'm The Juggernauts, Summon Thunders and souped up Rogues. This all stems from my experience in the adamantium league at around level 110 - 135.