I like the Yor. Exterminate all organic lifeforms!
I belive you mean "EX-TER-MIN-ATE!" (sorry, huge Doctr Who fan, and the Yor remind me of the Daleks for some reason.)
Anyways, my favorite race is a costom race I made, the Assryian Star Imperium. (Mouthful, isn't it?) Their story goes like this: Thousands of years brfore they took to the stars, they were at war with a reptilian race known asa the Ha'taks. It quickly became genocidal, and eventualt the Assyrains won, but the cost of vicory was high: the complete extermination of the Ha'taks. Or so they thought. They discovered an opposed tribe of Ha'taks known as the Skathalet, and took them in as equals. Now, in thier galactic emergence, they hope to redeem themselves for what thier ancestors did all those years ago.
P.S.: If costom races don't coun't, I'd say the Korx. Why? They're living proof of what can go wrong when you mix criminals and capitalisim.