One that fits your butt. Basically you want to have your weight on the sit bones, not the tissue in between. Seats come in different widths and shapes to accommodate different shaped butts.
Custom ones can be made after they scan your pelvic region. Essentially, there's a depressed, or cutout section in the middle with two pads in parallel which should support your bones as the other poster mentioned.
i don't know, i know i have a bad one, as after 15+ mins of cycling, when i get off the bicycle... can't describe it differently.. my balls sleep... and i can describe that that is not a pleasant feeling. e.g. 10 fold worse then a sleeping foot or leg.