iTunes, but a bit grudingly.
The media library (ie. list view + browser) is the best way i've found to view and play my music. Period.
It also leaves my files alone (if you've ever had problems with this, you need to change prefences) but has the advantage of changing the ID3 tag when you make an edit which means the changes stay no-matter where you play the file.
The reason why I have to grit my teeth a bit is because Apple have turned it into more than just iTunes which makes it bloated and sluggish. On Windows it also runs a load of things as services (Bonjour, iTunesHelper, iPodService, MobileDevice) without asking you if you want Bonjour or if you own an iPhone.
Oh, and Coverflow is one of the most useless things ever.