Personally, any nipple in my mouth is good, but I do like the big brown ones. On light or dark skin. I especially like those big pencil eraser nipples that grow when you bite them!
I like a well defined border on big aureoles, not ones the look like a busted egg yoke. I've seen many and I prefer them as compared to small ones that look more like a mans nipple. I don't care how big the boob is if the nipple area is small it's not near as sexy.