gas pistons...... because the right piston can make all the difference in accuracy (read gus fischer)
gas plug...... because ya never know when one might fall out in the mud/snow..... or wherever.
extractor, detent, spring..... probably the most commonly lost part if the detent fails these parts go into flight.
ejector and spring...... don't often lose the spring but if the extractor goes into flight..... usually the ejector does too.
spindle valve, spring and pin..... probably never need them but good to have spares.
same for the oprod guide pin as the roll pins have been known to collapse after a time.
couldn't hurt to have a few firing pins put away as well.
the rest is ammo ammo ammo....