Melakon: I thought of Dennis Hopper, and that's exactly how I'd picture him. Not a "great" Starfleet Captain per se, but the guy's whose gone past the edge, seen too much and said "f*** it."
(Crispin Glover perhaps as a junior officer who gets possessed by the soul of an ancient criminal.)
TheGoodStuff: I've always thought Jack Nicholson and/or Gene Hackman would be awesome Admirals.
One example I've missed egregiously is Jon[sp?] Hamm. Capt. Don Draper, so to speak, would be the perfect stand-in for Chuck Connors as an emblematic WASP Captain of the (twenty-two-)Sixties.