The ones most frequently recommended at videohelp are Media Player Classic (MPC) and VideoLan Client (VLC). MPC is a development of Media Player 6 by an independent project. Both have an onboard MPEG 2 codec, every damn configuration option one could think of, and can play buggered files when nothing else can. Both also have a very small footprint, no fancy resource-hogging graphics or splash screens.
I've used both, currently using MPC simply because it can be run from an executable (handy on a workplace machine, eh?).
Look here:
http://www.videohelp.com/tools?s=11#11