For me the most Underrated Gecko is going to have to be the entire genus of Oedura.
I first came in contact with Oedura monilis. I saw them at SEWERFest in 2009 at Phil Tremper's table. They were sleek black babies with neon yellow spots. The velvety appearance and just awesome body structure of this gecko caught my eye immediately. As I kept them I got to see them go through a pretty amazing change in pattern and colors. They became calico and patterned with random spots of yellow, with white borders and a black background. They go through a pretty drastic color change at night where they almost completely flush out their colors and the black becomes a lavender purple. They also have a lot of character. Mine will spaz out when sprayed with water, the female will eat dubias from my hand, and they are very inquisitive. Along with all of that, mine are relatively handleable.
Over the next year I plan on collecting pairs of every oedura species I can, within the next few months I plan on having in my collection, monilis, robusta, tryoni, and marmorata. I want to specialize in this genus.