For some time, my favorite has been the Storm spell Leviathan. I love the head peaking up from the water to check your location, and they swaying back and forth before the strike. Far from the most elaborate of the animations, but I find it to be very effective.
Second would be Deer Knight and its cousin Reindeer Knight. Creepy, those creatures striding out of the woods.
I must give strong honorable mentions to (1) any spells where snowmen are abused - that running joke really amuses me (inspired by Calvin and Hobbes cartoons, perhaps?), and (2) animations that start out looking like one spell, and then shift in an amusing way - the von Katzenstein monster grabbing the vampire bat after it leaves the coffin and using it to smack the target is one example.
The first time I saw this spell was the first time I got to Celestia, and I'd never played a Storm before (well, not to that level anyway), and my initial reaction was "Uh oh, what's this?" (That was the reaction after I got hit with it too, LOL). But the spell animation is beautiful. With the water floating up the way it does. It's a really cool spell.
I also like Reindeer Knight. It just brings a smile to your face to see the trees decorated.
