Depends on your definition of "young"--Johnny Depp is in his 40's, but he's got another 30 years in his career. Leonardo DiCaprio will be around until he's old and gray. Matt Damon is a phenomenal actor that crosses genres. I'd even say that Ryan Gosling has the talent, as long as he continues to make good role choices.
chloe moretz for sure! after i saw her performances in "diary of a wimpy kid" and "kick-ass", I'm convinced she's going to be the next big name actress as her career progresses. Trust me, she's that good. Not only is the young 13 year old versatile as an actress, but she has a great onscreen presence. That girl is going places, as she'll end up being a household name before you know it.