For me it was formerly Haxorus, but now I think it goes to Scolipede. The idea of a megapede is really neat, and it gets some really neat moves like Earthquake, Rock Slide and Superpower. And it got a slight Attack buff and Speed Boost in XY, which is so much fun. In case my current avatar doesn't tell you, I really like this thing.
I still do think Haxorus is cool, but after thinking it over Scolipede has taken its place for me.