Surprisingly, my largest crit was in a AB. Defending stables as a full Frost mage. in comes a Warrior and Druid. I of course say to myself " Crap, I am so done for", then I decide that I will at least get a shot or two off before the imminent death.
I trinket, Icy Veins, etc. Druid is charging me, while Warrior gloatingly caps the flag. I blink, land next to Warrior, FN, do a jump routine to gain a bit of distance, FB. The FB lands, 3457 crit. Warrior blinks at me in astonishment, Druid drops bear to HoT, and I maliciously sheep him.
I died like 4 seconds later from Warriors MS crit, but was a satisfying conclusion.