Butterfree can be great as long as you build him/her right and can strategize with their moves. Bug types tend to have many useful passive/non-lethal moves that with good strategy; are very effective. A butterfree with a powerful grass type move, accompanied by either stun spore or sleep powder is great. Adding other moves such as aerial ace to the mix for some diverse move types, and finally including a move such as a normal or possibly a fighting move is possible might be one of the strongest butterfree builds.