I have always stocked up on whatever I have used in the past, figure I will most likely need it again sometime. I do a lot of remodeling, some woodworking, metal work, auto, etc, basically a little of everything. So keeping supplies on hand adds up quick for me. I find that I have to or it's cheaper to order a lot of things online so I will order extra or different sizes to keep on hand. I think it all comes with time, you find out what you need from past trips to the store and stock up depending on how often you use it. Also I always seem to buy extra and have left overs.
I have stuff for remodeling, glue, nails, nails for all the nail guns, staples for all the different staplers and guns, screws of all sorts and sizes for drywall, decks, wood working, kreg jig screws. Plumbing supplies, copper fittings and pipe, pvc fittings for all sizes and pipe, glues and soldering supplies, wax rings, shut off valves, supply lines. Electrical supplies, outlets, switches, romex, metal and plastic boxes, staples, wire nuts, breakers, plug ends, etc. I got sick of running to the hardware store all the time needing a fitting or electrical box so just decided one day to spend around 400 bucks and buy all sorts of plumbing and electrical supplies which has saved a ton of trips to the store. If I want an outlet put in or need to fix a shut of valve I just walk out into the garage and grab what I need. Also has saved lots of trips because you always forget at least one thing when you run to the store to get all the supplies for a project.
For metalworking I keep grinding and cutting wheels, sanding discs, welding tips, cutting fluid, and a good supply of nuts and bolts in different sizes. Also have a bunch of different pneumatic fittings for projects I have built and lots of different air compressor and pipe fittings and accessories.
For wood working I keep tons of sand paper and nails and screws, replacement blades for just about every saw I own.
For auto, hose clamps, fluids, filters, shrink wrap, wire ties, nuts and bolts of all sizes, trim screws, electric connectors, loctite, gasket sealant, greases, brake and carb cleaner, etc.
There is way to many things to list or even remeber. Best advice is to just buy what you have needed in the past or think you will need in the future. Then if there is different sizes or kinds of something you need pick up a couple of each to keep around. If you need 5 of something, buy some extra depending on price.
The most important thing I can say from experience is to have some way to keep everything organized! Otherwise before you know it you will have stuff all over and not be able to find anything and just have one big mess. Shelfs and bolt bins of some sort are a must. I kept buying stuff and just set it places and before you knew it I couldn't find anything and the garage was a mess. Doesn't make a difference how much stuff you have if you can't find it. Then you have to go buy extra when you can't find the stuff you stocked up on.