I have: Iron Man* lvl 30
Black Widow * 30
Thor ** 30
Storm *24
Captain America Steve Rogers *** 18
Human Torch *** 15
The Hood **15
Venom 14
Storm Classic **11
Spider-Man Bag Man ** 8
Black Widow Original ** 8
Hawkeye Modern ** 8
Captain America Modern *6
Ares **6
Bullseye **6
Wolverine Astonishing **6
Juggernaut *3
Venom* 14
Spider-Man Bag Man ** 8
Hawkeye Modern ** 8
Captain America Modern *6
Bullseye **6
Juggernaut *3
Any of these are good candidates.
All of them are of fairly limited use for various reasons.
Juggernaut’s easy because he’s only at level 3.
Venom’s pretty much useless unless it is a villain / Dark Avengers tournament.
Bagman, Cap, and Hawkeye are mainly used for events like Heroic versions of the prologue, where they tend to be some of the only characters available.
Bullseye is of limited use unless you’re using him on a defensive team to piss people off. A defensive team of a maxed Bullseye, Daken, and someone else takes forever to fight because of their strike and defense tiles.