I've used Windows media player.
I've used VLC.
I've used SMPlayer (the gui frontend of mplayer)
WMP plays maybe 60% of the stuff I have. It's easy to use, and is user friendly.
WMP 12 (Windows 7) Is absolutely terrible, hard to navigate, unprofessional, and lacking.
VLC Player: plays everything I have, except FLV (if I remember correctly).. Settings are NOT user friendly. Has horrible quirks about it, such as resizing the video window every damn time a new video starts. Glitches the sound/video for a couple seconds while seeking. Pretty MEH interface. I would like to have a playlist that integrates with the main player, as well.
SM/MPlayer: plays everything I have flawlessly. The settings are a BREEZE to flip through. They all make sense. Nice options available, the interface looks good. However, this is not without cons. Because SMPlayer is just the frontend, it opens two processes.. the GUI and the MPlayer backend. This causes a slightly noticable delay upon opening the application, and interacting with the movie (seeking, etc). It also causes crashes when they get confused with one another and they both decide to crash. Another thing I noticed is that the keyboard controls are TERRIBLE. SHIFT + keys are required to change videos. Every other application known to man just requires N or something similar.
Verdict: I've stopped using WMP completely. I've stopped using VLC because of the quirks. I use SMPlayer right now, but as VLC improves I may switch back, because it feels more responsive. If the SMPlayer gui was integrated with MPlayer more tightly, and the keyboard controls were changed, I would absolutely keep it.
I apologize.. Slashdot is not keeping my data in paragraph form.