1.
I tend to use Winamp (5.xx) for most of my music and video playback. I'll use VLC or Creative Media Center in the cases that Winamp can't do what I need it to.
2.
I have also used:
Windows Media Player (Too clunky, too big, overly complicated, slow, memory hog, I don't like the User Interface. I do use it for ripping CDs though

)
Quicktime (Having Quicktime installed interferes with my default MIDI synth when clicking or autoplaying a midi file on the internet (quicktime takes over as my default no matter what I tried). Also, I saw nothing special from this player).
The KM Player (Seems decent but too complex for what I need. I never use sound manipulation inside of a player; that's what editors are for

. I want a player to look pretty, have a few customizable features, but not go overboard with them.
Foobar2000 (I didn't use it enough to really appreciate it. Mostly I didn't see a reason to switch from Winamp to this. The memory usage of Winamp isn't an issue for me (4 GB of RAM goes a long way most of the time) and I couldn't justify getting used to a whole new system to save a couple of MB of RAM.
VLC Media Player (Decent but not very pretty. While it is fairly useable, I find fast-forwarding and rewinding within a file a bit clumsy and setting up a playlist isn't as friendly as I'd like to see either).
Creative Media Center (Useful for one type of audio file: AC3. That's all I've ever used it for since no other player on my computer can properly play AC3 files. No visualization for frequency analysis. Not my favorite playlist system. All in all, not my favorite user interface.)
3.
I've stuck with Winamp because I picked it up many Many years ago and learned it, mainly. I enjoy the features such as the frequency analysis bars in realtime (up to 70 frames per second). Plugins seem to be available in large quantities for Winamp, including support for .nwc (Noteworthy Composer project files). I find it powerful enough and user-friendly enough to warrent the slight cost of memory which could be saved in other apps (See The KMPlayer).
I've got a 32.7 MB MP3 that's 14:18 in length. How would iTunes respond to that?
I've got one that's 183 minutes and 290 MB

(Techno megamix

)