I can't pick one, so here's a few!
Jake Gyllenhaal is a personal favourite. Mainly because he was the lead in my favourite film of all time, Donnie Darko. He's also been in quite a few other very good films considering he's only 30. Zodiac, Jarhead and the recent Source Code are probably the pick of the bunch. I've actually never seen Brokeback Mountain (cue gay jokes) but obviously he was in that as well.
Jackie Chan also gets a mention. He sometimes comes across as a bit of a crap actor in American films, but I think it's just his unfortunately comical accent. He's definitely got acting chops though, which he shows in loads of his native films. But mainly he gets a mention because he's an indestructible f*cking ninja.
That's films. Anyone who knows me knows I'm a TV junkie. I absolutely love David Cross, mainly for being Tobias in Arrested Development, and in that having possibly the best comic delivery I've ever seen. He nails every line so well I can watch the episodes over and over again and still laugh out loud at his parts.
James Spader is another RFUS favourite. I adored Alan Shore in Boston Legal and he also had a brief cameo in the US Office in the episode which showcased potential replacements for Steve Carrell. I thought all of them were a bit tame but he stole the show completely and as such has been picked up to be in the new season of the US Office. Admittedly that show is badly gash now, James Spader's silky voiced character will make me still tune in.
Kristen Bell, although now a film actress, was a favourite when I was addicted to Veronica Mars. Unfortunately since moving to the big screen she's been in some completely gash films, and always seems to get a shallow character. She's a great actress though and it's a shame to see her wasting in the movies. She needs a new agent!
Idris Elba is my final mention for now. He's just the f*cking bomb. As well as plenty of smaller things worthy of note, breathing life into Luther and Stringer Bell is enough to get him on any list.