Multi-flippy screwdriver thing. You can just about dismantle an entire playfield with this tool alone.
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2 size philips screwdrivers
2 size flat blade screwdrivers
1/4" Nut driver (pull the screwdriver blade and it's a nut driver)
5/8" Nut driver (same)
$4 - $7. You should have 8 of them. One in each car, toolbox, kitchen drawer, pinball shop...
Put these tools in a bag and you can deal with almost any pinball problem.
Flippy screwdriver thing
Adjustable crescent wrench
Needle Nose Pliers (with a wire cutting blade)
Digital Multi-meter
Soldering Iron
Solder sucker/wick/bulb and solder
I think you can build that whole bag for $40 with a trip to Harbor Freight and Radio Shack.