I answered Windows Media Player because I believe it's the one I use most often. I recently downloaded the portable version of VLC and installed it on a flash drive, more or less as an experiment. I used it to play a DVD and it was fine. For video, I have used or tested Windows Media Player, VLC, CyberLink PowerDVD (came with my DVD-ROM player), and I find them all quite adequate.
For music, I've used Windows Media Player, Creative MediaSource Player (came with my mp3 player), and Audacity. For playing, I believe I prefer Windows Media Player (probably preferred) or Creative MediaSource Player. For editing, Audacity is good.