The Fluffy Menace! (Prepare for a bit of an essay, because I know this is going to require some explanation.)
Altaria @ Leftovers
Trait: Cloud Nine
EVs: 252 HP / 4 Def / 252 SDef
- Dragon Pulse
- Roar / Haze
- Heal Bell
I know, I know, what am I thinking? At first glance, it seems like Altaria is outclassed in pretty much every aspect. Dragon Dance sets are inferior to every other Dragon Dancer - and every sweeper really - in the tier, it's bulk is actually not bad, but Latias, Dragonite, and even Salamence seem to perform better in defensive roles due to their abilities, movepools and/or stats. Since it's inception it seems to have been doomed to roam the lower tiers. But with the advent of BW, Altaria got something that really turned around it's viability in OU: Cloud Nine. This is a really anti-metagame ability, since it completely removes the effects of weather while Altaria is in play. This allows it to make deceptive use of it's bulk, since between it's key resistances and any weather boost to Fire or Water moves being removed, it can easily shrug off attacks. For reference, Altaria takes weather-boosted attacks almost two times
better than standard Latias.
Of course this sounds great on paper, but to understand how Altaria actually fares against weather we have to look at how it stacks up against common threats on weather teams. Here are some examples of how surprisingly beefy Altaria is:
252 SpA Choice Specs Keldeo Hydro Pump vs. 252 HP / 252+ SpD Altaria: 102-120 (28.81 - 33.89%)
+1 252 SpA Life Orb Volcarona Fire Blast vs. 252 HP / 252+ SpD Altaria: 137-162 (38.7 - 45.76%)
Yeah, it's that good. (Even HP Rock Volcarona gets phazed if Stealth Rock isn't up.) Just this alone is unique, and would give Altaria reason enough to have some use, but it doesn't stop there. I took a look at several prominent Rain and Sun threats, 14 to be exact, and Altaria is able to check at least some variants of 10 of them. Just to list them out, they are: Politoed, Keldeo, Gyarados, Rotom-W, Thundurus-T(Scarf, you should really have another response to this though), Tentacruel, Toxicroak, Ninetales, Venusaur, and Volcarona.
You might be thinking things couldn't get any better for Altaria, but there are still a few perks it has. To start, it isn't trapped by Dugtrio. This is huge, since it's quite common to see Dugtrio used as a way to eliminate certain weather checks. Heatran, for example, is commonly forced to use the awful Shed Shell in order to prevent combinations like GeneTrio from running it over. Altaria has no such problem. Second, it has reliable recovery. Using Heatran as an example again, although it's not particularly vulnerable to hazards, is prone to getting worn down quite a bit over the course of matches, often resulting in it being incapable of walling it's intended threats. Altaria is weak to Stealth Rock due to it's Flying-type, so this is crucial to its' success as a defensive Pokemon. Finally, it also gets some nifty support moves you don't see too much of in OU. I generally prefer to use Roar, but Haze is insanely useful against Baton Pass, which even well-built Stall teams often auto-lose to. Heal Bell is also really great, since it supports your team excellently, and prevents defensive variants of Ninetales, Politoed, and Tentacruel from wearing it down with status.
Unfortunately, there are some ways in which Altaria falls short. What stands out the most is it's 4x weakness to Ice; a lot of Pokemon wield these moves, and it seriously hurts our fluffy friend's walling capabilities, since there are certain threats - and variants of threats - that Altaria absolutely cannot hope to defeat. Altaria is also incapable of walling Genesect and Tornadus-T. This is quite unfortunate, as they're arguably THE offensive threats in the current metagame (I guess you can get lucky with a Hurricane miss against Tornadus, though). Stealth Rock also hurts Altaria some, as although it can still wall the threats in needs too, it will have a more difficult time switching in repeatedly. Finally, it also struggles with Dragon types, since they can easily pummel it with their super-powered STAB moves. This isn't anywhere near the end of the world, though. Specially Defensive Rotom-W, for example, solves three of these issues easily.
Here's a log of Altaria in action:
Possibly more to come.
In summary, Altaria is criminally underrated, and should at least be considered on teams ranging from Stall to Balance that struggle with weather.