VLC Player is the most straightforward, and the most reliable player you can find. I especially like the cropping functionality it has for 16:10 monitors, as it makes good use of your hardware. Performance is excellent, and pixelation is minimal.
DivX player is good too, but there's nothing it can do that VLC can't, and honestly it just takes up more memory, and it's kind of useless.
I use Windows Media Player 11 and VLC Player for all of my entertainment purposes. VLC is my failsafe.