Can't just choose one, so I'm gonna cheat.. I've whittled it down to... 6.
Actresses: Meryl Streep is amazing. She can take on any role and even if the script doesn't give her much to work with, turn it into a masterpiece.. Her performance in "Sophie's Choice" is probably the best acting I've ever seen, period.
Cate Blanchett is another fave of mine. She's v. versatile and never fails to convince.. "Notes On A Scandal", "Elizabeth" and "Heaven" are great dramatic performances, but she can also do lighter stuff like "Bandits" v. well..
I think Uma Thurman's v. talented too. It's a shame that she's done so many weak movies.. All the same, the "Kill Bill" movies (especially the second one) and "Tape" show that she's got a lot of depth..
Actors: Cillian Murphy's incredibly talented and deserves so much credit for his almost flawless career choices to date.. He's really got the mix right between Hollywood blockbusters ("Batman", "Red Eye"), and nifty indie films ("Intermission", "Watching The Detectives"). He's really gutsy and will take on anything he views as challenging- "Breakfast On Pluto" is a must-see..
Joseph Gordon-Levitt's a totally under-rated fave of mine. He's consistently chosen roles purely on the basis of their artistic worth, and hasn't really bothered about what's "commercial" at all.. He's given so many outstanding performances over his career to date that it's hard to pick faves.. "Brick" and "Mysterious Skin" defo showcase his ability though..
Christian Bale's brill too, even if he does have an obnoxious off-screen persona. I can't bring myself to watch "American Psycho", but he's been fantastic in more recent movies like "The Prestige", "3:10 to Yuma", "Equilibrium" etc. His commitment to his roles is pretty legendary.