Usually what breaks first depends on how hard the crash was. For additional spare parts I'd get main rotor blades, tail rotor blade (And tail motor if applicable) tail boom, landing gear skids, rotor head, fly bar rod, main rotor shaft, swashplate, and control linkage. It's hard to tell what will will actually get broke first, so it's a good idea to keep all these additional parts handy. Because it is possible to break all these parts in one crash. A good thing to do is, everytime you go to the hobby store just pick up a few parts everytime you go. And after while you'll have a lot of spare parts.