I've spent a fair amount of time with the stock Media Player, Canola, and Media Box; additionally I have MPlayer installed and use it for videos.

I prefer Media Box because it works really well for internet radio, which is what I use the N800 for most of the time it's in use. It displays the song-artist-album-etc metadata that come along in a stream, and it shows the rest of my favorite streams on the same screen, so changing streams is easy, quick, and thumb-friendly.

Media Player displays the metadata too, and it really isn't bad, except for its unappealing look-and-feel. Canola has a great look-and-feel, but it's rather basic in its functionality with streams and playlists.