What do you mean by Hiddleston has done surprisingly little?
He's just brought a TV series to huge success, he's just done Crimson Peak and High Rise, he is starring in the new King Kong movie, he did Coriolanus very successfully last year... he is having a very diverse career in terms of format and style. Independent films and blockbusters, TV shows and theatre.
Ruffalo has had two consecutive Oscar nominations in the past two years too, btw, and he only joined the MCU a few years ago, so not sure why you are suggesting he doesn't have much going for him.