Oh easy. The God among men that is
Stanley Tucci.
He is one of my all time favourite actors. He seems to slide by being absolutely perfect in so many movies.
The Lovely Bones, Captain America, Under Cover Blues, The Devil Wears Prada, Julie & Julia, The Hunger Games, The Terminal. I lub him. (I even
created a webpage devoted to him because I'm a big nerd)
Now, I admit Stanley doesn't always physically transform as often as many other great character actors do. One of my favourites that certainly does to to the point that people don't even realise he is in the movie is
Vincent D'onofrio
Da man.
Honestly I could go on naming people all day but I thought I'd pick one last one that might be unexpected...
Chris Evans
Yes he has been a lead in things, I mean he is Captain America for gosh sakes.
But I am always surprised at how many people don't realise Chris was also..

One of the 7 evil ex's in Scott Pilgrim vs The World
and

Jensen in The Losers
Maybe it's partly a level of fame thing, but I seriously think he is one to watch.