For OU, in addition to Justinawe's suggestion of Cresselia, Suicune is an excellent choice. Latias can also wall both ways pretty effectively, especially on a Dual Screen set. With Reflect up, Tyranitar's Crunch fails to 2HKO without a Dragon Dance, and Latias can always Roar it away if it tries to set up. And if legendaries just aren't your thing, have a look at Dusknoir.
For UU, in addition to Registeel, Claydol and Spiritomb are solid options. You might also try Shuckle, which can force quite a lot of switches between Toxic and Encore.