I don't know if I'd say undiscovered but for me it's Speedvagen. The term luscious always comes to me when I see one of these. Apparently the ride quality along with artistry is second to none. Definitely my next build when I can afford it.

SV.landing

From a pure utility standpoint, I know some folks who have had incredible success with the Nashbar and Scattante branded frames (aluminum and carbon). Don't know that I'd call them undiscovered gems, but certainly don't get the attention they may deserve simply due to many of our pre-conceived notions