As a Pokémon fan, I was so amazed with this Generation's games. They are so beautiful in many aspects; however, as a competitive player, I got very disappointed way too soon. I left competitive play during the later part of Gen V, so I didn't get to see exatcly why Keldeo, Alakazam, Technician Breloom and the Unova legendary trio's Therian-formes became such feared Pokémon, so perhaps I'm a bit too rusty to make accurate comments, but anyways...
Gen VI is looking to be, by far, the generation that introduced the most disappointing set of Pokémon. So far, the only Pokémon that seems to be unanimously expected to be OU is Aegislash. I was really hyped for Barbaracle and Malamar, which look pretty interesting and seemed promising competitively, but, at the end, all of these new additions turn out to be somehow outclassed by older Pokémon, making their niches "meh" at best. It even looks like the only competitively-relevant stuff happening this Gen involve Mega Evolutions, a couple new moves (such as Parting Shot and Sticky Web, which, so far, are getting pathetic distribution) and the overall nerfs to weather/special attacks.
I can't help but to be disappointed seeing how weak and unsurprising (bar very few examples) the new Gen Pokémon are turning out to be. Also, all of the somewhat interesting Pokémon from this Gen seem to be awfully slow, which just adds on to the fatalist-looking future for these new creatures.
Optimistically speaking, though, I can only see around ten Gen VI Pokémon, at most, reaching OU, even if it's only for a while:
1.- Aegislash - Most solid candidate so far.
2.- Goodra - Apparently, the new pseudo-legendary, but still outclassed by many other dragon-types.
3.- Greninja - Protean sounds promising, but those awful defenses don't.
4.- Barbaracle - Outclassed by Cloyster or not?
5.- Gourgeist - Interesting typing, perhaps a decent spin blocker, but nothing astounding.
6.- Malamar - Contrary is such a godlike ability, and it has some tools to take advantage of it, but its stats, Speed particularly, just seem too lackluster to realistically hope for a solid position in OU.
7.- Dragalge - At some points (and just like with every other Pokémon in this list), some user says something about him that makes it look great, but then, someone else appears to say something that makes it look like an UU Pokémon at best. Haven't tried him out yet, however.
8.- Talonflame - Looks solid with that incredible ability in Gale Wing, but it's just too frail and also has an overwhelming 4x weakness to SR. It can perhaps turn out to be a case similar to Volcarona's and get great performances with adequate support.
9.- Florges - Maybe? I haven't read much about her, but she is frequently mentioned... I don't know.
Every other Pokémon discussed so far just seem to be so painfully disappointing.