I really like the yuan-ti - because they embody the "fear of taint" that H.P. Lovecraft showcased so perfectly with his Deep Ones. Finding out that your lineage - or the lineage of a loved one - includes ancestors that were less than human makes for a nice atmosphere of horror.
I liked them so much that I made them the rulers of a major region in
Urbis. And then, of course, they were removed from the SRD, which means that I can't use them if I want to publish the setting.
Bummer.
I've got a few ideas for a substitute - a race of psionics (norma, untainted humans
cannot learn any psychic powers in Urbis) that use psychometabolism to breed with
any terrestrial creature (i.e., living beings that aren't extraplanar or aberrations) of approximately their size. In southern regions, snakes remain popular for their breeding experiments, but now they can prosper in other regions as well...
Apart from that, I like hobgoblins - a goblinoid race that isn't dumb, and is highly organized. In Urbis, they are a major force with their own kingdom and available as player characters.