If we're going with full birth cycle generations of approx. 20 years, I'd go with the Strauss/Howe years throaty mentioned, and allow some movement for those at the bookending years to the next or prior generation, so '61-'82 with exceptions granted. That works for me, because there is no way Johnny Depp is not an X'er in spirit, and he would have graduated high school in the '80's. Clooney's arguable either direction, and Penn, though born in '60, exudes more of the X'er ethos to me than Boomer. And if he's included in the Gen X category he goes right into most people's top 5.
Others for submission:
Michael Fassbender
Benedict Cumberbatch
Tom Hardy
Matthew McConaughey (some will laugh, but then they'll see Dallas Buyer's Club and True Detective, and we'll all realize it's too early to even have this conversation in some senses).
On the other hand, we could categorize "actor generations" as something different entirely, as alluded upthread. I'd argue that within 5 years either direction, give or take (so 10 year total) around the actor or actress to whom the phase "of their generation" is being attached, works fine, with some flexibility based on when an actor/actress hit their stride and became well-known. Because a lifespan in the world of entertainment is so much shorter than in life, so movements come and go much more quickly. By that standard, Sean Penn is clearly in the Tom Hanks generation, then, along with Denzel and others, but George Clooney is not. In any event, this is a smaller group than the traditional social studies idea of a generation. That yardstick takes people like Gosling and Ledger out of the discussion, as well as the additional people I've mentioned above, since they didn't even hit their strides until after someone like Hoffman, who had early success, had already won an Oscar. When we say that Philip Seymour Hoffman was one of the finest actors of his generation, then, I think what we're referring to is "amongst people who started garnering acclaim and a lot of notice in the mid- late-'90's and then sustained excellence for some time thereafter." That's a significantly smaller group than just Gen X'ers generally.