In terms of competitiveness, there's a lot, to be honest. First and foremost would be Cresselia. Not much people use it (which is sad, in my opinion), so unfortunately, it doesn't usually get called up to over-used tier, but when generation four was still current, I'd utilize this Pokémon quite heavily. Thankfully, it's never managed to let me down. It's a great tank, all-in-all; its Defense is huge, and so is its Special Attack and Defense. What's even better is latter two could also be raised, thanks to it having access to Calm Mind. With Levitate, it's immune to all ground-type hits, and due to it being able to learn Moonlight, the Pokémon is able to recover itself whenever it wishes.
Second would be Jirachi. This Pokémon has Serene Grace ability, which allows it to take full advantage of Iron Head move that it's able to learn--flinching foes. Once they flinch, they're completely restricted from touching you for a whole turn. Jirachi could use Thunder Wave, too, I believe, and if you have it on its move-set alongside, chances of your opponent landing hits would be even lowered. As for third, that would most likely have to be Shaymin-Sky, due to similar reasons. It's banned, I know, but it could still perform quite well in ubers when given the chance.
Design-wise, I'm a big, big fan of Yveltal. To me, it seems like an evil version of Lugia, which is absolutely cool. Lugia, itself, is very nice-looking (to me, anyway), but seeing a dark side of it, so to speak, is more or less astonishing. Along with these two, few of my other favorites would be Darkrai, Deoxys, Giratina, Xerneas, Rayquaza, and Zapdos.
