Porygon-Z, for being perhaps the ONLY pokémon you could give Hyper Beam without being called a n00b. With it, I have managed to single-handedly destroy uber teams three-on three wise, and have won contless battles with it.
Next in line, there's Porygon2. Porygon2 is notable for being one of a handful of not fully evolved pokémon that are still able to be used above anything other then UU, to the point where it can demolish pokés such as Heatran and Gyrados with little problem. Porygon2 is perhaps my second most powerful pokémon, first if you count level-wise.
So go go, Porygon Squad! ^.^