I chose FOOBAR 2000 solely for the audio conversion. Not just straight audio files, ie. mp3 to wma, or to ogg.
But with video files, if I demux them, I can decode DTS 5.1 and convert to AAC 5.1
or I can convert to WAV then run it thru WAVtoAC3 encoder, and end up with
5.1 AC3 audio.
Skinning FOOBAR is tricky I find, but can make some fascinating looking players if you are willing to put the time into learning how.
As well, there are MANY, MANY plug-ins for it.
Other than that, I might be interested in XION.