I would definitely say top 3, in order are Sunset, Scaleless and Monarch, as mentioned.

However while they are the most rare and most valuable, that doesn't mean they're the hottest. In terms of most popular despite volume, that would be Clown, Leopard and GHI right now. Coral Glow/Banana are still going fairly strong, but they took such a major price hit when the volume exploded, there are plenty of them just sitting right now. In all four of these morphs, there is still plenty of unexplored ground, which is where you really want to be.