Anyway, glyph envy aside, let's consider the contenders. The header image is a pretty good clue here -- I love the look of a Frostfire Bolt! The blue frostbolt encased in fire is a seriously lovely effect. But I do think there are other abilities that rank very high. Penance, for example, has a really cool-looking effect. The bolt itself isn't that amazing, but the way it makes those spiralling paths toward its target is very pretty indeed. I also think priests have a damn fine Divine Aegis bubble right now. What else? Well, Chaos Bolt looks great. That little face -- I love it!
I sense melee players are feeling rather unloved. Hmmm, tricky. I do think the paladin Avenging Wrath wings are pretty sweet, especially when coupled with a Badge of Dominance's red effect. A cool effect doesn't have to be a traveling effect, of course, and I think the DK Anti-Magic Zone looks pretty great too.
I remain undecided about my favorite, although I am leaning toward the Frostfire Bolt. (There's a reason it's in the header, after all.) So persuade me! Throw your vote into the ring. Do you agree with me, or am I completely wrong? I'm often completely wrong, you know. It happens a lot. What's your choice for the best-looking spell effect in WoW?