Dialga @ Life Orb
EVs: 4 HP / 252 Atk / 252 SpA
IVs: 0 Spe
-Outrage / Thunder
I copy pasta'd this from Theorymon's Project Ubers thread. If you want to read a bit more about it (though, I'm assuming you already understand the concept) or check out other under rated/used sets that's a cool place to check.
[01:47:47] <+Limi> gamefreak has confirmed the rumour
[01:47:53] <+Limi> that mewtwo now has a tumor
[01:47:59] <+Limi> but please man, chill out
[01:48:03] <+Limi> you don't need to pout
[01:48:08] <+Limi> just take it all in good humour!
Originally Posted by Fat trickroom
Blizzard is for the whole Dragon Slayer thing, it OHKOes almost any Dragon in the tier save Kyurem, Giratina, Dialga, Palkia, Reshiram, Zekrom, Latios, Latias and Giratina-O.