My absolute #1 is Steve Buscemi. From "Resevoir Dog's" to "Charlotte's Web", he completely embodies the character he's portraying. He obviously imprints them with his own person, but that is unavoidable with any actor, and his "own person" is why no one else can play the roles he does.
My 2nd pick is Michael J. Pollard. Terrific character actor that disappears into most movies he's been in. "Roxanne" with Steve Martin is the most well known movie I can think of that he has been in.
I take just a "little" exception to Jeff Goldblum being scoffed at, in that one of my favorite movies stars him, "Mr. Frost". In fact, I just recieved a VHS copy via Amazon because I couldn't find it produced on DVD.
Bill Murray: Caddyshack is classic lowbrow (maybe the crowning achievement!). He completely transended that legacy with "Lost In Translation".
Nikilidstrom: Hey fellow Michigander/Red Wings/Lidstrom fan! I completely agree with Rutger Hauer. It so disappoints me when great actor's either can't get work or choose not to work. "The Hitcher" is the last movie (besides some awful sci-fi vampire flick) I saw Rutger in.
Although not quite the same, two other actor's (specifically the "acting" aspect) that have squandered their last good acting years (IMO) are Clint Eastwood not doing any western's since "The Unforgiven" and Harrison Ford eshewing the pure action roles that made him famous in favor of playing CIA/the President/some other "official". It's sad that he held out until this late date to play Indy again.
Jewels443 and Charlatan: Everytime I see Bill Paxton, I can't help but think of "Wierd Science".
Love him in "Big Love" though!
Ali