They're not really in order of importance, but all are nice to have on-hand.
They're all considered replacement parts, but some are more prone to wear-out faster based on their function.
If there were one "extra" part to have on-hand, I'd probably say get an extra bolt. It's an easy part to swap-out, and it's small enough to carry within the buttstock or pistol grip of an AR-15 (provided you have a compartment in either one).
I hope this helps.
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