Heliolisk

With a base 109 Speed stat, Heliolisk outspeeds a lot of common Pokemon such as Infernape, Galvantula, and Cobalion. It also has a great movepool, boasting moves such as Surf, Grass Knot, Focus Blast, Thunderbolt, Hyper Voice, Volt Switch, and Dark Pulse. Heliolisk also has Dry Skin, which provides it an immunity to Water-type moves and therefore allows it to threaten a multitude of Water-type Pokemon in UU. It also has a nice immunity to Ghost-type moves, meaning it can switch into Choice-locked Chandelure and nab a free Volt Switch.

While Heliolisk has two immunities and a colorful movepool, it is weak to common attacking types in UU such as Ground and Fighting. This is also exacerbated by the fact that it has pitiful 62 / 52 physical bulk, meaning even resisted physical hits are going to take a chunk of Heliolisk's HP. It must also use Life Orb, which can wear it down even more quickly if it wants to take advantage of its coverage moves.