What would it be?
I used to know this guy who did a podcast for his web series. One day while answering listener questions, he read off a really fantastic question related to World of Warcraft.
"If you could have any ability in WoW, what would it be?"
At the time he picked Teleport: Shattrath, rationalizing that with that spell he could teleport to Azeroth and learn the rest of his mage's spellbook. A very "wish for more wishes" kind of answer, right? Eventually his co-host foiled him though, reminding my friend that a Rune of Teleportation regeant is required to cast the spell so he'd never be able to cast it on Earth.
I thought long and hard on this question myself though, and being prone to illness I was immediately drawn to a paladin's Cleanse spell. Never being sick for the rest of your life would be amazing, right? Not to mention, you could cure everyone else. I don't know how Cleanse would respond to viral infections versus bacterial infections, but I'd really love to knock out HIV. I don't know what mana cost would do to a person in real life, but I bet Gatorade is just like mage water. I wonder what level I am?
Anyway, in the end, my boyfriend at the time brought my attention to an even better idea: Concentration Aura. As a creative sort, I imagine that the aura would be useful for getting mundane things done. Like starting that back burner novel, or studying for a test with your outline group. If you were performing something on stage, all your fellow performers would receive the benefits of the aura as well. Imagine working at DARPA with Concentration Aura up: we'd destroy the world in a matter of years. I mean... what?
So what ability in WoW would you pick for yourself?