I'll always recommend Russet potatoes and specifically small or medium-sized (about 1-4 inches) for this recipe. They'll cook faster if they're not too big, but still large enough to hold a good amount of filling. The skins on a Russet potato are coarse and sturdy so they won't break easily under pressure. You can also use Yukon Gold potatoes if you prefer. Some people appreciate the more delicate skin and prefer it to Russets, so the choice is yours!