Definitely Someplayer ( http://talk.maemo.org/showthread.php?t=70405 )

I need the ability to play from folders and in addition to that, Someplayer simply has the best, most natural sound quality of all the players that I have tested (including the default player and Symfonie). It also features (among many other things) an equalizer, although I never (have to) use that.

The player is a bit tricky to get started with, but once you get the hang of it, it actually is not really difficult to handle.