So, expired Lockboxes are available again (I hope you loaded up at the last gold sale), and everyone wants to get the most for their gold. These are general guidelines on which LB characters are currently most useful. As always, you decide for yourself who is best for your game. For my part, I highly recommend the top two characters, as I currently use them regularly. I would qualify the bottom 6 characters as good only for completionists.
1. Magneto : Good damage, preemptive, shield as a QA with Snappy Service in play. Few characters have held up over an extended period of time like Mags has.
2. Sandman : Highly playable character with lots of damage. One of the few true first-round one-shot artists.
3. Loki : He requires thought to play well, but dat turn advantage.
4. Omega Sentinel : She remains very versatile, with one of the game's largest skillsets. Whether you take her on the attack or use her to set up your defense, you can get a lot of use out of OS.
5. Juggernaut : I'd say nothing can stop him, but I've stopped him a number of times. Even so, he's pretty scary to see on the other side of a PvP match.
6. Satana : I might be overrating her just because I like her in group boss battles. But I really do like her in group boss battles. That L2 turns your whole team into health-devouring attack monsters.
7. Thane : Blaster class bonus, KO's bruisers on his first turn with his L1, debuff prevention, stuns... the list goes on. Thane does a lot of things well.
8. Taskmaster : For a guy who takes extra actions and has built-in counters to all class bonuses, he's rather disappointing. But still, he takes extra actions and has built-in counters to all class bonuses.
9. Doom : If they'd fix his health he would actually be useful in PvP. He has the skills and the build to be a difference-maker. That said, using two characters is generally better than using one.
10. Elektra : I like stealth, I used to like bleeds. Just forget she has a preemptive, as you'll probably never see it actually proc.
11. Sabretooth : Makes no sense in the story, makes little difference in a fight. That said, you can do worse than berzerking and an AoE stun.
(Note: Several commenters have asserted that Sabretooth rates too low on this list. To that I say: my blog, my rankings. But for your game, do take into account the possibility that every other MAA player in the world likes him more than I do.)
12. Moonstone : Her turn sequencing is actively bad, her big attack is too small. A real disappointment after all the hurt she's put on my bruiser Cap in 1.12.2.
13. Avalanche : While Avalanche boasts a couple of interesting game mechanics, they're generally not good enough to actually use.
14. Kang : Just doesn't pull the weight of two characters. After accounting for his teamup restriction, he's generally a liability in battle.
15. Constrictor : From time to time he'll drop a Chrono Boost and a Double Strike on the same turn and go off. But those times are pretty rare. Usually he's just too slow, and always he's too inconsistent.