Oh I did a meme not too long ago where I had to sort various characters into Hogwarts houses. Anyway, my short version would be this:

Natasha - Slytherin
Clint - Hufflepuff
Steve - Hufflepuff
Thor - Gryffindor
Tony - Slytherin
Bruce - Ravenclaw

Here be more detailed thoughts I used in that meme of mine; and I'm bit lazy right now (and also have to leave house really quickly, but I want to share all the thinky thoughts!)

Steve - he is a brave soul, but Huflepuffs are brave as well - they're just not loudly brave as Gryffindors are. He is honest, loyal, hard working and genuine; his heroism is of a quiet and humble kind; all of that belongs to Hufflepuff house. He can show compassion, he dislikes unfairness (think - his answer to the question of he wants to kill Nazis - that's typical, typical Hufflepuff), and most of all, you can rely on him. maybe it helps to compare him with Thor, who is a Goryffindor poster boy. Or Ron Weasley, or Fred and George. Gryffindors have a bit of recklessness in them, and I don't think Steve does. He is a peaceful man, his primary motivation isn't the fight, isn't heroism, it's righting the wrongs. His weapon is a shield. His loyalty is fierce but selective; think of his reactions to Tony - it takes a while until Steve is convinced that Tony is, in fact, a good guy who would lay himself down on a wire. Steve is a tough one, I think, but when he said "I hate bullies I don't care where they're from", that solidified him as a Hufflepuff for me.

Tony - a Slytherin poster child. He is definitely the best that Slytherin house can offer - smart, cunning, picks his fights, ambitious, powerful, classy. He's arrogant, but that arrogance is a mask, beneath is a good man who will do good things. He just doesn't want to be called a hero; and he doesn't want it because he doesn't believe he is one. Oh, he does like flash and glitter, but that's different than being a hero. That's hollow stuff, beneath his classy, fine, playboy bravado is a lonely man with daddy issues who feels so very alone. I heart Tony because I can relate *points to icon* We Slytherin people are tough on the outside, and very soft on the inside. I heart how he reacted to Bruce - he was the only one to approach poor man and face him, mostly because Tony knows how it feels to live with a monster inside you, only Tony is Iron Man's monster. Slytherin loyalty is selective, but once it's given, it's forever, no matter what. When everyone thought they'll never see bruce again, Tony kept asking, in the middle of a battle, "Is Banner here?"

Clint - could make the sorting hat take a minute or two to ponder where to put him - maybe. Someone might say that he's a Gryffindor because he doesn't even bat an eye during the entire fight in Manhattan. That guy is fearless in action, but he lacks the loud, I'm - a - star mentality of a Gryffindor. he rather wouldn't be seen; he prefers being up there, observing, essentially, doing his work. So, Hufflepuff. Is Clint nice? I think he is, once he gets to know people; he is definitely less approachable than Ryan or Carson, but I can imagine three of them getting along rather well. I think he's the example of the toughest that Hufflepuff house can offer; I also see him as genuine and honest, and someone you can rely on, but his loyalty isn't free. It has to be earned, and you can see that aspect nicely hinted at in his interactions with Natasha. You just have to wonder what happened between them to make their bond so solid - I love that scene when he wakes up, and Natasha is there (obviously!), and whatever they have, it's incredibly solid. Her devotion to him is strong and even painful at times, his loyalty to her is steady and, I think, endless, and it's amazing how he recognizes her during their fight even while he was under Loki's control. (I just heart them! And I better stop blabbing about them, because this might turn into a shipper manifesto).