Steve:
His plaid button downs - He's always confused as to why they keep disappearing until he sees you wearing them over your own shirts.
Bucky:
His Sweatpants - They're big and super comfy. The perfect clothing item to wear while cuddling, on the couch or in bed.
Pietro:
His hoodies - They smell like him and are good for cuddling, which is why you usually steal them before he goes on a mission so you don't miss him as much.
Loki:
His nice button down shirts - He thinks they look incredible on you the morning after. You love keeping them when he's away, because it's like you're have a part of him with you always.
Clint:
His sunglasses - they look absolutely ridiculous on you, but you steal them to make him laugh and it works.
Thor:
His cape - You like to steal it and run around the apartment imitating him. He finds this quite entertaining and laughs hysterically at you, which is the whole reason you do it.
Tony:
His band t-shirts - You enjoy the same music he does and his shirts are super comfortable. Also there is the added bonus that they smell like him.
Natasha:
A pair of her leather pants - When you first stole them it was joke to make fun of her, but instead it turned her on and you decided to keep them.
Wanda:
One of her red jackets - You both think it's cute when you match, and she loves the way it looks on you.