My most commonly used fasteners for prints are m2 countersunk self tapping screws like these.

They’re small, but generally stronger than the prints they’re going into. If it’s something that may need to be disassembled and reassembled, I’ll use M3/4/5 socket-head cap screws and nuts or inserts.

I also keep a variety of magnets on hand to imbed in prints.

Then, of course, there’s bearings, threaded rod, aluminum extrusions, hinges, hookup wire, electronic parts, clear acrylic sheeting, etc.

My usual scheme for purchasing hardware goes something like this:

Create design and bill of materials > buy a bulk pack of hardware that includes the specific parts I need, plus extra if it’s cheap > store extras in 3d printed modular drawers, adding more drawers to the stack as needed.