I know it may not be in the spirit of the question, but my personal favorite ability is my "oh ****" kit.
I have macro that hits Evasion, SnD, Blade Flurry, and Adrenaline Rush for when I over aggro or get bored. It also spikes your dps to something absurd for a minute. Its a lot of fun.
Warning: The makers of the "oh ****" macro DO NOT endorse its use for all activities, including the following: raiding, 5-maning, arena, BG or anything but soloing and ******* around.
Seriously though, it was extremely usefully from 55 to 70.
SO I guess in a roundabout way that makes Blade Flurry my favorite