Here is what I keep in my spare parts box.

main shafts
feathering shafts
tail drive shaft
main gear
tail boom
blades (both main and tail)
canopies (not mandatory...I like having a spare and also quick change of the looks of the heli)
servo rebuild kits

Just about any crash or hard landing will cause you to bend shafts (sometimes and more often than not all 3)...so IF I were only going to stock the bare essentials...it would be the shafts.