Spot lights 16ga
10GA with a fuse right at the battery, connected directly to the battery. Power lead (well insulated) is then run into the headlight bucket and a sealed automotive relay (1 input; 2 output terminals) are used to switch power to the spotlights.
Individual feeds from the relay are 12GA. Relay can be triggered via a variety of switching methods. Mine are controlled by a SPDT center-off switch that follows the Lo beam, is Off or allows the spot lights to be On constantly. (Check your state laws. I'm G-T-G everywhere with this setup.)
If using a light bar to mount the spots, run a separate frame ground into the headlight bucket so the steering bearings won't pit due to being part of the ground circuit. I also use 10GA for this run and it's as short as possible.