Personally, I like the old-style Predator tanks, with the dome turret at the back. It really does something for the look, and I'm glad FW brought it back with the Deimos. Right now, I think the Sicaran is actually my favorite design, being the ******* child of the Land Raider and Predator. Something about those huge track pods just screams "awesome", and the amount of lead it can throw for as cheap as it is (points, not the resin itself T_T) is pretty sweet, too.
Also, Knights. The design of the old epic Reaver is one of the things that caught my eye and drew me in to 40k, and the plastic Knight nicely echoes that shape, in a form that's manageable in price and size.