For me, it was Spine. imho - the concept of that fight is better than any other I've done. So, I had such a fun time doing it.
Prior to that, the fight that sticks out most in my head is Sarth 3D. When I first tried it (before we tried 1D or 2D), it blew my mind away. The complete chaos reminded me of the first few times we tried HKM.