Up until recently, my music library program of choice was Windows Media Player.
However, then I got an iPod Touch which requires iTunes (iirc, one of the alternative programs for iPod Classic/iPod Nano has iPod Touch support in a nightly/beta, but apart from that, you do need iTunes).
My biggest problem is speed, my music library of 30GB on a network drive via Wi-Fi, is never going to be kind on any media player, but iTunes can at times take up to 2 minutes to respond to a click. And this is with CPU/RAM usage nowhere near full.
Is there any way of speeding up iTunes?
EDIT: Most (if not all) iPod compatible programs are NOT compatible with the iPod Touch/iPhone. Please double check that your program is compatible before suggesting it as an answer.