I'd say Clowns and Pieds are the top two hottest that will always be in demand. Why? Because they blend well with other genes. Some people chase the newest genes like Rainbow, Acid and Sunset, mainly because they are newcomers to the market and pretty cool stand alone morphs, but I'm not too sure they will combine with other genes to make as many impressive combos as Clowns and Pieds (time will tell). I think the ability to blend to make a variety of beautiful impressive combos is what it's all about.

The Highway / Freeway is a niche market, makes a lot of really cool combos, but unless you have supers it's hard to hit a whole clutch of them. Plus when you breed a Highway to a normal you can't tell the difference between the yellow belly and Gravel / Asphalt babies, they all pretty much look normal, another spin in the combo that turns people off (I think).

Scaleless is probably the next big hit coming down the pipe. It basically reset the market and now you can do everything in scaleless, I would say that's another great investment with long term potential. You can pick up scaleless head normals now for about $1,000 and the price is coming down as we go, so not so out of reach of most people any more. You can breed them to your old normal females and get a pretty nice return from those normals!