Really, as an Alt-Holic I can say that the best feeling of finishing someone off is with Lava Burst and Its cannonball sound, Not Starsurge, No Pyroblast, No Arcane Barrage, No finishin moves, no execute. Lava Burst.
If I can land a killing blow on a Disc Priest, with ANY ability, it feels pretty damn good. Having said that, my favorite is probably a windfury proc on my shaman. It isn't nearly what it use to be, but it is still possible dammit!
Heroic leap, crush the squishies.
Heroic throw, the squishies think they can run away?
Anything spell reflected, because it's funny.
Bear hug, just because.
Snake trap, because they are so weak and its fun to throw snakes at people.
Basically, things that shouldn't be used for killing; utility that can do damage.