A bunch of EM Guys here. I'm a little surprise there hasn't been any response yet.
Pop Bumper and Flipper parts for rebuild. An assortment of #47 lamp sockets. A
couple On/Off toggle switches I myself like to have on hand. Extra of those hair pins..
You have a Majorettes?

A junk playfield and bottom board of cabinet, both still having most of their parts.

Then you have a great supply of parts you can scavenge.

LTG : )

Well, at the risk of stating the obvious, above all else you need lamps, rings, fuses, and coil sleeves. These should be new, not used. You probably want to have a few new coil stops on hand, too.

A few bumper rings are good to have on hand, and again, it's usually better to buy these new than to put in used ones, mostly because they are somewhat labor-intensive to replace.

If you can acquire one or two salvage/parts machines from a particular manufacturer for a specific era, that should supply you with all the used lamp sockets, switch blades, relay coils, solenoid coils, coil stops (if not worn out), fuse holders, wire, playfield posts, hardware (screws and such), and stepper parts you need to keep a few games going. You'll also have one or two back-up score motors and transformers, although these rarely need replacement.

Once you get into the plastic parts, probably the most important thing to have on hand are bumper skirts, and again you might want to consider buying a few new ones. For other plastic parts, you can get into the weeds pretty quickly because there are a lot of plastic parts and a lot of them are machine-specific. However, things like flipper buttons, star rollover centers, kick-out hole bottom plates, and lane guides are good to have on hand.

The number of pinball parts you might want to keep on hand if you are doing in-home repair work tends to multiply quickly. For example, standard U-shaped lane guides come in several colors, lengths, and two different form factors (one-side wall or two-side wall). And, the style varies from one manufacturer to another. Or take bumper skirts. These come in several different colors, and at least two styles (one style for passive bumpers, and another style for pop bumpers).

So in a way this is a fairly non-trivial question if you are trying to cover several vintage eras from all manufacturers. Focusing on one era, and one or two manufacturers, would make your parts stock less complicated.

- TimMe