Tony -

He's always looking at the way your clothes tightly hug your curves

Steve -

He loves staring into your eyes to see what colour they are as he claims they change every day

Clint -

He watches the movement of your hands as you write

Bruce -

He can never keep his eyes off you as you train with the other Agents

Thor -

He stares at your legs when you run

Loki -

He loves watching the different faces you pull whenever you react to something