Insomnia
Seriously, I don't understand why everyone's voting for Air Lock. You are all aware that its effect wears off once you switch out, right? I don't see how the ability can benift him in anyway, apart from Leftovers recovery, which is hardly a point to consider. You have absolutely no business staying in against Garchomp, and you'll hardly be able to touch Gliscor, making evading Sandstorm hax useless. Please enlighten me why you'd ever even consider using this ability.
Insomnia is a great ability, and the vast majority of you seem to be underestimating it. The ability to switch in on a Sleep move for free is sorely underestimated. And who's to say this Poke can't adopt a slightly more offensive set to go along with this ability? You guys complain about not being able to use Rest, but don't forget you still have Shed Skin. Apart from Sleep Talk, and the rather ineffective Natural Cure, it is very difficult if even possible to counter sleep. It can often cripple a Poke for the entire match. Sleep Talk isn't even a surefire way to deal with Sleep, due the probability of selecting Rest or another useless attack, thus losing a turn. Switching Natural Cure Pokes in and out does nothing to stop the Sleeper from coming back and doing its job later.
Think of Insomnia as an immunity to the most crippling status there is. And a 100% one at that. Besides, whose to say we can't give this guy Pain Split, when like half the Ghosts get it. Not to mention his rather uncanny resemblance to the Duskull line. Definitely a breeding move.
I seriously believe Insomnia would open a more offensive avenue for this Pokemon, allowing it to fulfill more than one role and become more surprising, something which definitely can't be said for Air Lock. Imagine switching in on a Scarf/Specs Gengar/Roserade's Hypnosis, then Focus Punching their switch-in off the face of the earth. You might think this rather situational, but more than half the time the first move a starting Gengar, and Roserade to a lesser extent, uses is Hypnosis. And most of them are Choiced.
Ultimately, we seriously need a Poke with Insomnia in OU to effectively deal with Sleepers, and frankly I'm surprised so few people realize this.
I bet half the people voting for Air Lock don't realize its effects wear off after switching, and are only supporting it because they think it'll help them deal with Garchomp.