Venomoth, by far.
Butterfree was fine up until you got to Vermillion City, but pretty useless after that due to the stronger Psychic types that became available, and Beedrill is just awful, generally. Pin Missile and Twinneedle are decent moves, but Beedrill can't land more than one hit without being KO'd.
Venomoth is one of the fastest sleepers in the 1st generation, has a wide and varied movepool and can cause all sorts of problems with its unpredictability. It's fragile, but it's not built to tank or sponge hits, it's like a Bug-type version of Electrode with less speed, very much a one-hit-and-switch Pokemon.