Tornadus- it doesn't get nasty plot, but it has prankster or regenerator (though you lose either one once you mega), as well as U-turn, heat wave, and focus blast.
Articuno- blizzard, U-turn, signal beam (not that useful, but cool), and ancient power (also meh).
Moltres- (so much better than articuno) everything articuno has, -ice stuff and Sbeam, + fire blast, fire spin, and WoW.
Volcarona- basically just a better moltres, it gets fire blast, hurricane, QD, U-turn, WoW, and giga drain.
Yveltal- definitely one of the best, it gets oblivion wing, psychic, snarl, focus blast, shadow ball, hyper voice, and dark pulse (Also heat wave and U-turn, but let's assume from now on that everyone I mention has them, cause they basically all do and I'm sick of typing).
Swanna- surf, scald, I've beam, not much else. (No heat wave or u-turn. Probs the worst)
Castform- actually a pretty cool option, gets fire blast, hydro pump, thunder, blizzard, and a BUNCH of others that I don't want to take the time to type out w/ 100% acc. (No u-turn)
Whimsicott- Giga drain, Moonblast, shadow ball, grass whistle/ sleep powder, a TON of other status moves, etc. (Another of the best).
Noivern- boomburst, Draco meteor, dragon pulse, shadow ball, focus blast, super fang, dark pulse, a couple other cool things... (classic OM Noivern).
Vivillon- basically, volcarona, (-anything fire and giga drain), w/ sleep powder, stun spore, draining kiss and psychic.
Last but not least:
Dragonite- possibly my favorite, it gets Draco meteor, dragon pulse, thunder, fireblast, blizzard, surf, and focus blast, + Multiscale. It has no means of boosting SpA, though, sadly.
Oh also, turns out Mewtwo gets it by event.
Mewtwo- psystrike, almost every TM, a bunch of other stuff. Hands down, the best choice (unless you are running a grass whistle set or something).