Hmm, it's not easy to decide. I really enjoy drawing turians from Mass Effect franchise, and quarians are nice too. I also like "pretty male" races like elves or catboys or fae or etc. Sylvari get a honorary mention, I only drew them once (twice if I count a current work in progress), but I love the race. Then there are demonic creatures, not heavily featured in my drawings but monsters and demon men are hot.
I don't think I can pick a 100% favorite, but just by the number of drawings I guess turians are technically the winners.