What are the most common part on that break on a Glock?   I have two Glocks and nothing has ever broken but I know eventually everything that moves brakes down.   I have but many, many thousands of rounds through my G22 without a problem, not more than a few hundred through my daily carry G27.   I keep spare parts for my AR15 but none for my Glock.   Which would you recommend?

i keep a spare of slide internals, trigger assembly, but not pins or mag catch.  I have extra complete slides and sprinds waiting to go - sucks to get to the range and need something (I had a broken extractor in my G19 first range trip).

cross pins that get lost
recoil spring assemblies
internals around firing pin - nothing worse than losing a small cup or other part
  when doing a full cleaning.

Make sure that you Glock has not qualified for an upgrade.  If it is qualified, it is not that just one part is replaced, a few are replaced at the same time at no charge.  When is a free upgrade not a free upgrade?  When it is really a factory recall at no charge and buy another name.

The foregoing is just for normal servicing.

If  . . . shtf, an administrative order banning imports goes into effect, there is a run on spare parts, etc., you need to anticipate that as well.

The guy who took over Glockmeister or any of the usual parts dealers that you come across on this board will help you fill out the rest of your list.  By the way, there is no retail dealer in California that carries spare parts to my knowledge, so don't count on going to the store and buying spare parts.

Recoil spring assemblys for sure.

My first gen G17 had a totaly spent spring, and a busted guide rod when I got it. It also still worked broken! I shot it a bunch the day I got it, then went home to discover the guide rod was bigtime broken!