I don't know about my most badass ever, but recently, I'd say my lvl 70 Crobat in B2 with Fly, Acrobatics, Poison fang, and Crunch. Lvl 63 by the time i got to the E4 and had pretty much no trouble at all with them at that point.
Someone call Kenny Loggins, because we're in the DANGER ZONE
Even as an Ice/Flying type, this thing was a tank. Even a Rock-type move couldn't OHKO it.
In particular, I remember one battle in Battle Revolution, where my other two Pokemon were both OHKO'd by an Arceus-Dragon. Out comes Articuno, and it single-handedly beats Arceus-Dr, Dialga (who's immune to Toxic), and half a Palkia, just by wearing them down with Ice Beam and Toxic. Judgement, Spacial Rend, Roar of Time took, like, 1/3 of my HP. Articuno just doesn't give a ****.