I've used many, as the OEM tank wasn't enough for a commuter. On the front drive MitsTLE43 tank, I'd literally have to run up a tree to fill the tank.
I've tried goped tanks in series with the original tanks. These leaked a lot, I misplaced the cap often, and caps are not universal fit. A 1-gallon tank came with my Subaru. It was ok, but the fuel line was wrongly placed. Once the grommet came off, and I got fuel spilled down my butt.
I used a 2-liter Peanut auxiliary tank to feed my twin engines. Since the front engine was higher, I needed a 12-volt fuel pump to finally keep the tanks full.
My best tank has been the 4-liter Peanut tank, either mounded on the top tube or on a rear rack. You can raise it a bit to clear the top tube's cables. Now I've got a gymn bag over it for stealth. The locking gas cap is priceless.