Quoted By:
Poliwrath. It's been a total bro of mine since Red Version, and I've trained one in pretty much every generation (often the same one, but I started getting into breeding around Gen 4).
It's UU, but I honestly don't see why. With the right team support (for example, Trick Room or Tail Wind), I can switch it in when I'm expecting a Water attack, fire off an Encore, and get in a Bulk Up or two. By then the other guy decides to switch, so I go with Circle Throw. Of course, the opponent wants the Pokemon he switched in before, (only rarely will they NOT switch again), so I go with a Scald, which never fails to burn, and then Circle Throw again and again.
I'm not a Showdown guy, so this is all Wi-Fi competitions, and it never fails to get a ragequit or two.