I got tired of waiting on parts from tower or making trips to the lhs so I bought a couple of almost everything that I thought might break. The most common stuff was:

Main shaft

Feathering shaft

Tail boom

Skids (both the landing gear and the skids) The skids themselves are changeable and I havent broken any through use, but if you follow the manual and place glue in this area for the training gear then good luck reusing them if you break the gear itself which has happened)

Blade grips - oh and more blade grips, I keep like 3 sets on hand and replace a set when I break them or make an order.

Spare flybar

1 spare canopy. Fitment isnt so great, always rubs on the main gear and sometimes tears in this area during impact. Also where the grommets are down low has a tendency to crack through and then you have nothing to hold it on well.

Main gear

Spare hardware set, nuts screws etc this stuff likes to play hide and seek

Spare linkages and control arms off servos (these like to bend and I have just been bending them back until one finally breaks.

Most commonly replaced parts for me are blades, grips, shafts without a doubt