I'm gonna have to with Jumpluff, here. It's not overpowered, and thought it's gimmicky, it's bearable.
What makes Jumpluff deserving of the title is, in my opinion, the furor around it. It almost single-handedly triggered the entire Sleep debate, as well as forced everyone to reconsider their definitions of competitiveityand viability. While it's a terrible Pokémon, Jumpluff stands for a lot more than Jumping Under My Parasol, Leaving Unique Free Forks. It represents the line that people are willing to draw between RNG and hax.
Oh, and it's a bit cute.