A couple years ago, KiWi made a topic asking what media player we all used. Well, my answer has changed since then, so I figured I'd rehash the question to see where we all are.
Back when that thread was made, I was using Windows Media Player as my main such program. However, while I don't really dislike it, I've moved away from it mostly because of an annoying bug I experience with it, where if I open a file and then immediately close it, it stays open in memory as an unkillable process that remains open for an indefinite period of time, and prevents WMP from being opened again until it closes. So if I accidentally open the wrong file, it means I have to wait until it finishes… well, whatever the hell it's doing, before I can open the correct one.
Aside from that, though, I've always had trouble using it to play media file types such as OGG, FLV, FLAC, and Matroska, even when it should be able to.
As I use those sorts of files more often these days, my main media player is currently Media Player Classic – Home Cinema (MPC–HC)—specifically the version that comes bundled with the Kawaii Codec Pack. I use it as my normal media player for must audio as well as FLV, OGG, and Matroska videos. I'll also routinely use it to watch other video types, but WebM and MP4, the other major formats I use, I have set to open in Google Chrome by default.
I will also occasionally use foobar2000 to play audio files, especially continuous albums that use a CUE playlist to separate tracks. However, while I still like foobar2000 for its functionality, I experience extraordinarily high disk usage by it when playing back files, and I can't really understand why, considering no other media players I use have the same problem.
I also have VLC installed, though I despise it, simply because until recently, I've used it to convert Monkey's Audio files to FLAC. However, as I've since found a foobar2000 plugin to open and convert such files, I no longer actively use it for any purpose, and will only resort to it if other media players for whatever reason fail to open a file, just because it's nice to have options.