While there are plenty of similarities among magic systems in MMOs (hey, how about you cast another fireball? There you go, champ!), there is a lot of variety as well. I'm always attracted to systems that put some though into their design, such as Guild Wars 2's illusion-based Mesmers, the mix-and-match Necromancers of Vanguard, and Lord of the Rings Online's wordy Rune-keepers.
At the very least, I have to applaud developers who at least put in the effort to gussy up the same mechanics in a new outfit. But when a team eschews the tired magic tropes and starts to get imaginative with spellcasting? That's when I perk up and pay attention.
What's your favorite MMO magic system and why?