Just so many to mention! I think I'll just end up going reviewing every fight, but off the top of my head I'll always laud Ichigo vs Ikaku as one of the greats.
A lot of the fights during the Rukia SS Arc were great, and a lot for the similar reasons.
I loved that there were sword physics, epic twists and turns and cuts, and man the wounds felt really raw. Just the fact that Ichigo complaining about having a bleeding cut over his eye made it all the more intense!
When Ikaku revealed how his Shikai really worked you felt the shock at the twist. When Ichigo finally got fast enough to catch Hozukimaru you felt the epicness.