There's just so many great musical moments, I like em' all except maybe for that rock song whilst Uma is breakdance/fighting (took me out of the scene by making it way too 'comedy'-ish).
My faves are the haunting use of 'The Grand Duel' in the Anime sequence (powerful stuff).
Yea, O-Ren & the gang in slow-mo is supreme.
'The Flower Of Carnage' after the fight with O-Ren.
'The Lonely Shepard' in the sword scene and end titles.
Can't wait for what QT gathers up for the next one's soundtrack.