While some sites have their own tiers, most people use Smogon's "Uber" tier. The Uber tier, contrary to popular belief, is not entirely Legends, and some Legendaries are not considered Uber. The Uber tier is currently under construction due to massive changes in Gen. 6, but here's a list of Uber Pokemon, as of Gen 5: http://www.smogon.com/bw/tiers/uber
