I'll actually go with George Clooney. I've found a good majority of his films to be underrated/undermentioned (O Brother Where Art Thou?, The Thin Red Line, Up in the Air, The "Ocean's" series, Syriana, Fantastic Mr. Fox, Good Night and Good Luck, Michael Clayton, Burn After Reading, The Men Who Stare At Goats, Solaris, Intolerable Cruelty, Leatherheads, .etc...).
A case could be made for Johnny Depp as well. Many of his best films are overshadowed by the blockbusters and such. What's Eating Gilbert Grape, Finding Neverland, Fear and Loathing in Las Vegas, Blow, the Rum Diary (new), Donnie Brasco, Ed Wood, Sleepy Hollow, and even the Imaginarium of Doctor Parnassus are all great films, but a lot of people wouldn't be able to say that they've seen them.
Honorable Mention: Ewan McGregor and Jude Law (I like your choice).
BQ: I watched The Last of the Mohicans a couple of days ago. It wasn't the first time that I've seen it, though. Pretty decent film.
BBQ: I consider every actor that I mentioned underrated, at least as far as actual acting skills go. You would be hard pressed seeing a question on here about one of them (or a discussion elsewhere) without seeing a population of girls talking about how sexy they think they are.