Steve: He likes to play as Luigi.
I don't really have an explanation for this, but it feels like this would be Steve's pick.
(He submitted his own, and I couldn't find out why he picked it.)
Tony: He likes to play as Mario.
He is the main character. Tony likes to feel like the main character.
Clint: He likes to play as Toad.
He thinks Toad looks funny.
Bruce: He likes to play as Bowser.
He feels like Bowser, because Bowser is a loud, big monster.
Thor: He likes to play as Princess Daisy.
Princess Daisy. The hair. It fits.
Loki: He likes to play as Waluigi.
Waluigi seems like a very mischievous character, and Loki is the God of Mischief.
Bucky: He likes to play as Baby Luigi.
He just had to pick a different character than the others, but he wanted to be like Steve.
Peter: He likes to play as Princess Peach.
Peter is a beautiful princess and nobody can convince me otherwise.
Pietro: He likes to play as Princess Rosalina.
I actually have a good reason for this, and he does as well, I swear!
She wears a blue outfit and she has beautiful light hair.
(Pietro is always my princess too!)