The latest build of VLC is stable after a slight wobble. It's handled everything I throw at it apart from hi-def rips and VP7 format files.
I like MPC, and MPC home cinema handles hi-def (e.g. Matroska) files very well. My only gripe, and it's almost a non-gripe is that MPC needs to switch Aero off to work. (EDIT - just MPC that does this oddly enough).
GOM player is pretty good, but doesn't do anything extra I can't do in VLC.