• R/B
• G/S
• R/S
• D/P
• Strategy
• Moves

Butterfree

Have it as your starter and use Sleep Powder on their first Pokémon, then follow up with Stun Spore as they switch to another. Continue paralyzing when they switch, and try to attack when they don't. Double Powdering requires some prediction (i.e. you Stun Spore a Pokémon, and use Stun Spore again, while your opponent leaves it in. They think you'll switch, so maybe Stun Spore for a third straight time, and catch a Pokémon that thinks it's getting a free switch-in). Mega Drain is there for healing and for dealing with Water, Rock or Ground-types.

Other Options

Supersonic is better than most of its other options, but it is inaccurate, and therefore dangerous to Butterfree. Reflect can work but, with Butterfree's already low defenses, is most likely a wasted move. Double-edge or Hyper Beam can be useful against Psychic types that Butterfree has trouble damaging.

Opinion

It's almost impossible to name a Pokémon that doesn't have a super effective move against Butterfree. It's basically a weaker version of Venomoth with a worse typing, and almost as bad as Beedrill in terms of attack and defense. It doesn't get any good STAB moves and it can't take many hits. If you put something to Sleep then it's done its job; it's not good for anything else, really.

Counters

Psychic-types and Chansey can almost completely wall Butterfree. Fire-types, Flying-types, and Rock Sliders can also usually KO it with one or two attacks since their attacks are usually super effective against Butterfree. Almost any Pokemon that can handle Psychic or Mega Drain and then hit Butterfree hard is an effective counter.