Although I've just discovered the joys of Jake Gyllenhaal, I really admire:
Meryl Streep
Jack Nicholson
Robert De Niro
Ryan Gosling
Heath Ledger
Penelope Cruz
Morgan Freeman
Nicholas Cage
Jody Foster
Sean Penn (got to meet him a few years ago, very nice man)
And a bunch of other folks. Why? I like to see actors and actresses stretch themselves, take risks
with the roles they choose, create a sense of really knowing the character they play. Heath Ledger was probably one of my all time favorites. Even though he was quite young, he brought a depth to each character he played, even before Brokeback Mountain. It was a darned shame he died too soon!