Definitely Robin Williams, he gave so much to every role.
Definitely not Robert de Niro. He was absolutely shite in Meet the Fochers but then it was a shite film so I think he was just going through the motions tbh.
Hugh Grant is an excellent actor in both light hearted and serious films.
Also, Emma Thompson. She is a bit like HG in that she can absolutely play the part well and has massive talent that she puts forward to whatever part she plays.
Although, I do think, like with HG and ET, they are quite astute in only accepting good roles, most of the time.