The best format to use is the one which will enable you to find what you want most quickly. And that depends on your own personal preferences and needs. Also depends what player you're going to use as well though because most of them will display the album and artist when you load up a track.
Personally, I'd be inclined to store them as
artist/album/artist-album-track_number-track_name.mp3
Have the best of all worlds