I use WMP for the most part, for a few reasons:
1. It was the only media player I had before iTunes was installed.
2. It does everything I need it to, and works with the special keys at the top of my keyboard for media (itunes only works with these when it is in front of other windows, however WMP works perfectly even when in-game TF2, allowing me to change songs etc without alt-tabbing)
3. It looks decent, and has an interface I can navigate without much difficulty.
Although I must admit, the new visualizer for iTunes 8 is very, very awesome looking...
So for me I guess it boils down to:
iTunes if I feel like watching a cool visualization.
WMP for everything else.