Frenzy Plant, Blast Burn, Hydro Cannon, and Thunder were always really cool to me because of how it made the opponent's Pokemon go into negative color for a moment. That's why Tri Attack is so cool as well. It combines Fire Blast, Blizzard, and Thunder with that cool negative effect.
Some other awesome ones are Rollout, Brick Break, Fly, Superpower, and Giga Drain.