[@Polybius]

I'm not a fan of selective eugenics or being the bad guys. However, my second pick would be a race of silicon-based organisms. In other words, living rocks, or to be precise, a race of creatures with hard outer shells that grow with age and have very long lifespan, at the cost of low birth rate. As such, they are not big fans of direct warfare, preferring self-perfection and arts.