SilverAngelus

Honestly, I think this will be a very quick matchup. Mega Beedrill is so overtuned that it should be able to outspeed Giga-Butterfree even after a Max Airstream (assuming Gigantimaxing doesn't buff the pokemon's base Speed)... but it wouldn't be able to even take in the first place, so it'd have to fell the powdery behemoth in a single shot.

TL;DR: Something's definitely gonna get OHKO'd in this matchup~!

Edit: It turns out that Mega Beedrill's base Sp. Defense is much higher than I thought- it's only 10 points lower than butterfree's Sp. Attack. So butterfree is actually completely screwed in this matchup if Gigantimaxing doesn't increase its Sp. Attack at all, which... is actually a highly concerning prospect.

I was thinking Mega Beedrill, but I can't say for sure only because we don't know all the aspects of Gigantamax and what it'd do for Butterfree's stats.