Since Bogart, Mitchum, and McQueen are no longer with us, I think I can claim that Clive Owen is the maximum supercool movie actor alive. If you've only recently discovered Mr. Owen thanks to Inside Man, or maybe Sin City, see if you can't hunt up Croupier, a little noir gem in which you get to hear his real accent. Or maybe even a highbrow movie with him, like Altman's terrific Grosford Park.
"Wait ... his real accent?" you ask. Yeah, Clive Owen is a Brit. (His name is Clive, for goodness' sake.) I'd say he has the best-ever American accent by a British film actor, too, were it not for the immortal Bob Hoskins.