Since you seem to be playing HP-MT, Blissey is a very good choice. Any variation will do. It's fast, reliable, powerful, and I love it. If it wasn't for Gallade, I'd be playing it right now. But since you don't have Gallade in HP-MT, then...
IF you have SW, G&G is the play! Banette and Magmortar are also good choices, but G&G, or Plox, is really good.
If you have GE, not much difference. Still Banette, Magmortar and G&G/Plox.
If MD, I don't know. Eeveelutions, Sceptile/Leafeon, Fossils, ect. are all good. We also don't know 20 cards in the set, so there might be something game-breaking there (Kingdra DP5, anyone?).
Other good choices are Infernape, Garchomp, Empoleon, Honchkrow, and Lucario. They aren't Tier 1, but they'll do.
EDIT: And if you don't have GE yet, Cessation Crystal is a VERY good Trainer.