Since RBY, my favorite attack has always been Sky Attack. Competitively, it's one of the worst attacks in the game. In-game, however, it wrecks everything that doesn't resist it, not to mention in every game has totally SWEET animation. It's always good.
Now that it's not a TM anymore, Sky Attack is only learned naturally by six Pokemon; three of them are legendaries. Another five get it as an Egg Move.
To those not in the know, Sky Attack is the strongest Flying-type move in the game at an astonishing 140 base power. It has 90 accuracy, which is the same as Hyper Beam/Giga Impact. It only has 5 PP, but that's fine since you'll never live to use it more than two or three times anyway. No one uses Sky Attack because it's a 2-turn attack. Unlike Hyper Beam and moves like it where you blast them turn one and recharge turn two, Sky attack charges turn one and blasts them turn two.
In competitive battling, this is stupid. Your opponent knows what you're going to do before you do it and a smart battler will be able to plan accordingly.
There's also the problem of missing... boy does that suck.
With the addition of Ruby and Sapphire, Sky Attack was given a 30% flinch rate. It's good, but still not good enough to warrant using Sky Attack for anything other than slaughtering things in-game.
Sorry I wrote like an essay, but hey - I loves my Sky Attack, and I'd be willing to bet not many people are too familiar with it.