My favorites are Thor and Ms Marvel. Thor plays exactly how I would wish him to play and I don't feel that he's too slow like Spider-Fan thinks. I have no issues dodging and he has a very good parry that links into power attacks. He hits hard with backup of his lightning (which helps you automatically parry while charging a bigger hit, and has great added effects to the hit) so I never have any issues getting off attacks that stagger or launch enemies, allowing me to be very offensive in melee, and throwing Mjolnir is a very satisfying mechanic, including pinning enemies under the hammer. He's strong both melee and ranged, grounded and flying, so he's the most complete hero in the way I play.
Kamala is less complete due to her limited range in ranged attacks, but on the other hand she has great melee range and can reach mele enemies before they reach her. Especially her unlocked airborne heavy combo is great to hit enemies without melee being able to hit her. She can also Embiggen a long time with upgrades, and her healing ability is really strong and makes her likely to be my favorite hero to have in my team beside me.
I think all heroes feel good though, except for that I have had issues with Hulk. He is the one I feel is slow and is less defensively capable, plus that jumping up on things can be a hassle. He'll probably be better when I play more and get the hang of him, but this far there's a reason why he's my lowest leveled hero. His HARM Room training mission was by far the hardest for me.