Well, they're all NU. If you want a Bug or a Flying type, there are better candidates.
I personally rank them in this order:
They all have access to Quiver Dance, but Vivillon has access to Hurricane and Sleep Powder, which will be near 100% accurate with Compoundeyes. Butterfree is second because of Compoundeyes + Sleep Powder, which means a near perfect accuracy Sleep Power. Mothim next because it can run a Tinted Lens set, but is way too frail and doesn't have Sleep Powder to set up. Then Dustox because it lacks any powder moves, but it's got a half-decent typing. Then Beautifly last because it's got crap abilities, stats and a subpar movepool, all on top of poor typing.