Tstire/Tube/Pump/Patch Kit/Multi-Tool w/ levers.
Skip one of those and you'll eventually be stuck with a flat and calling for a ride home.
Patch Kit is needed when you double-flat, find out the spare tube wore a hold in itself, or was a crap tube out of the box or just don't get out that piece of metal from the tire that keeps popping the tube. Small Pump is needed when you double flat, don't get a good connection on the valve, etc, etc.
CO2 is nice, but many times I've had to pull out my pump and help someone on the side of the road because they relied on it. Tubeless it's probably necessary tho.
-Steve