There's no "best."

Cantis or v-brakes both work fine, parts should be available fairly widely. Both work fine in the wet. Cantis are a little more adaptable to a wider range of road levers without hacks like travel agents.

Discs work well, as long as you have proper braking technique. Parts may be a little harder to come by, so if you're doing a really long tour you'll likely benefit from carrying spare brake pads. They're not in wide use for touring bikes yet, so going disc will narrow your options a little bit.