after playing it, this is my line up:
1) the strongest & most versatile hero is The Human Torch, due to the following reason: has a skill that knock downs many opponents per hit. (what is down, cant hurt you); is a flyier; immune to fire;
2) Iceman - 2nd to Human Torch in crowd control; slows enemies.
3) Mr. Fantastic - is very good for one-on-one or boss enemies; using his boost in critical hits creates devastating damages.
these 3 heroes (for me) is the strongest in the game. I used to play and think that Captain America was good, that is until i tried these 3.
try it.