Extra sling & flipper rubbers, flipper bats, flipper rebuild kit, bulbs, balls, and the coil for the winky target. That should get you started anyways.

Go through your owners manual and identify which type (amp) fuses your game uses. But slow burning versions of the types of fuses that you will need, five of each type. If you buy them from pinballlife they will be shipped in small metal tins and are quality American brands. Some other online retailers sell generic Chinese made fuses.

Molex connectors and pins are always good to have around.

In threads like this I always recommend the excellent database that oyvindmo made.
I looked at the parts most prone to breaking and numbers used in the games, and bought 1 or 2 based on that.
Fuses is a must have. Same with flipper rubber. And most recently I bought a ton of optos. You never know.

http://www.moll.no/pinball/parts/

Almost forgot! Assortment of screws and fasteners.

http://techniek.flipperwinkel.nl/wpc/
see: 1c. Getting Started: Parts to Have On-Hand

Extra opto interrupters for flipper buttons are a must, if your games have them. Preferably black.

I know this is more expensive but it always helps to have a spare, driver board. And I would recommend a new Rottendog, WDB089.

Also, used WPC DMDs are getting cheap ($75) a nice spare to have on hand.