I think the main criteria has got to be what you can actually fix, no point in bringing a spare piston if you can't realistically change it! However, things like wheel bearings, steering head bearings, etc, are fixable on the side of a road if you need to and could bring you to quick and abrupt stop if they fail - and given that they are relatively small and light might be worthwhile bringing.

The next thing is to bring spare parts for items that could actually fail, no point in bringing a part if it is highly unlikely to fail in the first place.

And finally the part isn't too heavy or bulky.

Ditto for service and wear items if you are going to be away from 'civilization' for a time (although in Aus you could realistically get most of what you need in any decent sized town so I wouldn't bother).

So my three main criteria would be that;
1. you could actually fix it,
2. part has been known to fail and if it did could leave you stranded, and
3. the spare is not to heavy or bulky.

So what should be on the list for an XT660Z?