If you're going to keep collecting, and not just keep a few cabinets and stick with 'em. I'd say monitor parts, fuses, etc etc.
Commons... they'll be most of your problems.
Know what companies sell what parts... like with B&D, I know that if I have to order anything from them... to tack on a couple packs of fuses because they've got a decent price break at 10 fuses. So, I stock up there on fuses, but only when ordering other parts.
My go to source for pretty much everything else is Mouser. I keep capkits saved in "projects" there so I can order them easily.
Keep a cap kit on hand for every major monitor that you'll probably run into. They're only a couple of bucks a piece and they're good to keep on hand for a late night CL run for a game with "monitor issues". 4900, 4600, K7000, Sanyo EZs... stuff like that. For G07's... keep everything on hand. While I don't have a spare flyback, I do keep HOTs and width coils on hand... ... same with 4600's if you can find a good stock of width coils.
The cap kits are nice because, if you take good notes and remember what you pulled... then you can cherry pick out of them for board repairs for games that might have a cap broken off... just make sure you always replace what you picked out as soon as you can... can get messy sometimes if you don't.
Besides fuses and monitor parts... like was said above, double/triple/quadruple orders of any 74XXX logic. They're ALWAYS good to have on hand and most are dirt cheap.
Diodes. Get a shit ton of over powered diodes. They're dirt cheap (I got a roll of something like 100 for a couple of bucks). You can use them to, well... replace diodes. But also replace bridge rectifiers in a pinch and stuff like that.
Any oddball parts your machines might have... they usually cost more, so just pick up a few spares when you can afford to, but don't go out of your way to make sure you have them. (long armed micro-switches like on some pins and Ice Cold Beer... for example).
As you come across "junk" boards... harvest them. Z80's, RAM, EPROMs, even PROMs in case you get that board later in a project. ... I'll even take the DIP switches off of them because I'm THAT cheap.
Wire, connectors, pins for those connectors, leafs, micro switches, LEDs, a good selection of resistors and smaller pots to ghetto replace resistors while you order in replacements (adjust the pot to the value of the resistor and run jumper wires)... it's dirty, but it'll hold until your new parts arrive. Ballasts, are another big one that you never think you'll need until you do. I don't know about everywhere else, but the cheapo $5 ballasts that Lowes and Home Depot used to carry aren't available here anymore. I lucked out and stocked up before they vanished... so lucked out there.
Oh, and sockets. Even if you have to order them by the gross from China, get a good stock/selection of sockets in all common sizes, then just get some crazy sized ones like 28 pin narrows... in a pinch you can cut them down to whatever size you need.
And I think I'm about done rambling... sorry about that.