chip question

[Kitemay]

what's the best defensive chip can be used on busters?

[HeraldOfOpera]

There are only 3 defensive chips to begin with, so we might as well just go down the list:

Mad Nautilus: locked to Sprayers so not actually relevant here.

Sting Chameleon: gives a big ol' pile of invincibility. What else do you want?

Giant Mechaniloid (serial number here): gives a stacking temporary damage reduction. If you're getting hit enough for this to do anything useful, invincibility is going to be better. That said, you have two weapon slots and want to have different chips on each for the stat bonuses so Sting Chameleon At Home still has a niche, especially when there are things that need specifically a melee weapon to kill so that second weapon probably isn't a sprayer.