What i use really depends on the kind of file i'm trying to see/watch.
Usually it's MMJ 6.10 for mp3 files, and ms mediaplayer 10 for other things, sometimes quicktime after stripping out itunes, i don't want that on my pc anymore, especially with that pesky ipodhelper that seems to be always on and a memory hog.
Strange that apple is allowed to ship qtime and itunes in one file, so you can't get one without the other...