It kind of depends on the situation and what I'm wearing on top. Under comfy clothes, commando is the way to go. Thongs are great for no panty lines or when I want to taunt my lovers.
But mostly, cotton bikinis are what I wear - they're comfy enough. No satin or nylon or synthetic fabrics - that's a yeast infection waiting to happen. I'll wear them if they're going to be removed very soon. Heh. I also like wearing men's underwear - Jockey low-rise briefs, to be specific, in dark colors. They are SO comfy - they don't ride up like most women's underwear and they're naughty in their own way.