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.)
Oh, there's another great Dennis Hopper villain I forgot--
Captain Howard Payne: Pop quiz, hotshot. There's a plasma bomb on a starship. Once the ship reaches Warp 6, the plasma bomb is armed. If it drops below Warp 6, it blows up. What do you do? WHAT DO YOU DO?!