My biggest rush is a tossup:
Getting that 1st place in a large field tournament after a prolonged HU battle where I've been a serious chip underdog at least once. I know it's luck but I love to tell myself it's skill

-- or --
Running really well while multitabling 6max limit. Bang, snap, ooh that had to hurt, oh my did I win again?, ooh don't bluff me man...
Best rush I ever went on was at party 5/10 6max, back in the glory days (Mar 2004). I won 250 big bets four tabling over the course of two hours and decided to just end it there for the day and go out drinking. BOOM HEADSHOT! That was during the brief period where poker was my only source of income and it was a good day.