Stranded 22AWG seems to be what all the major manufacturers use, and it's what I use. Bends easily; doesn't break, and it's the perfect size to easily crimp into just about any kind of crimp terminal. You don't need to oversize your ground bus unless you're also powering things off of it. Most games use 1mA at most for the switch signals, so 22AWG is just fine. If you're also using that ground bus for powering things like lights (I usually run separate dedicated bus if possible for various reasons), then you will want to upsize your ground to the same size as your power wire for the lights. I usually use 18AWG (still stranded) for this, though 22AWG is perfectly sufficient to run several lamps, especially with dedicated ground busses for power and signals.