It's hard to pick, but if I had to I'd probably say water lilies. I think the shape and the soft color of the petals looks very beautiful above the surface of the water. (I also like lotuses for the same reason, and actually I like the wider and more curved shape of lotus petals better, but the center of a lotus looks like the spout of a watering can and that looks kind of silly.)
Hm... I wonder... maybe something like the Sea Anemone, which isn't really a flower but takes it's name from one? Gardenia is nice as well but quite pitiful looking once it's petals start falling Also the Carnation is pretty and uncomplicated.
ps- this topic just reminded me of a flower I've been meaning to look up, the Nadeshiko flower.