Others have said brakes, and I agree 100% this is most valuable. I’m assuming most here already know these skills, so I am answering this in the context of what would I teach someone who doesn’t know how to work on a car how to do.

I previously would have put oil filter/oil next on the list, but you can often get that done for a couple bucks more than the oil and filter itself from a dealer that wants to offer to fix 10 other things on your car and plans to make their money on those. It’s still worth learning, but I would put two simpler tasks ahead of it these days:

For the easily daunted I would add changing air filters & wiper blades. When I do have to take my car to the dealer I always see them showing other customers their dirty air filters, and offering to replace them for well over 2x what a new air filter would cost, even though the labor would be identical to put back the old filter or put in a new one. Wiper blades are the same way, and the labor is literally a few seconds. They typically mark up the filter/blades so their labor costs don’t look as outrageous, but if people understood how simple they were to change they would never pay the outrageous prices. I really don’t even consider changing these “repairs” but since the dealer will charge you through the nose for them they are things you could easily teach any non-car enthusiast friend to do for themselves and save some money.

For someone willing to get their hands dirty, I might add spark plugs/ spark plug wires. This is another case where the parts are dirt cheap (unless you have ignition coils) and while for some engines they can be a pain to access, many engines are very simple to access and you can complete the repair for ~$10 if you skip wires, and under $50 even if you include new wires. Plus you can usually do the whole repair with a single tool (some newer cars have extra stuff in the way that has to be removed) making it easy to loan the necessary equipment. You don’t need to do this very often, but for a friend on a tight budget driving an older car it may be a good one to share. (I recently had a buddy fail smog due to ignition issues, get shocked by the repair estimate, and then borrow a tool and with minimal direction he changed them himself and was happily back on the road quickly and cheaply)