The fast version. Two ingredients, five minutes, done. This isn't technically "true" fluff (that's the gelatin-based version above), but it looks, feels, and tastes just like it, and it's the one to reach for when you're in a hurry.
Mini marshmallows melt faster and more evenly than regular-sized ones, so they're your best bet here. The corn syrup is what keeps everything soft as it cools; skip it and you'll end up with a sticky, hardening mess instead of fluff.