What houses would each of the avengers be in?
I would say that Steve is a Hufflepuff, first of all. He values loyalty, compassion, and kindness above all other traits, moreso than the typical soldier values of bravery and honor in combat. That's what separated him from the rest of the candidates and why he was selected to be the test subject for the super soldier serum.
This is going to sound probably typical, but to me, Tony is a textbook example of a Ravenclaw. It's seen in his genius, his snark, and in his desire to gain as much knowledge as possible. This also leads to a huge bit of skepticism, hence why he hacked S.H.I.E.L.D in the Avengers. His anxiety in Iron Man 3 lead him to further using his knowledge as a shield.
Natasha's Slytherin, through and through. She's ambitious, resourceful, and determined. She easily lies and adapts each of her stories to better manipulate her opponents or to get the information that she wants.
Bruce is harder to sort, but if I had to pick one, I'd say Gryffindor simply because he pretty much reminds me of Neville.