I'm going to back the Githyanki, Githzerai, and Illithids, largely because of the interactions between the three. It's genuinely interesting, they all feel genuinely fantastic where elves and such don't anymore, and it's actual competent fluff, which generally doesn't happen. There are also some constructs I like, particularly the Inevitables and the gigantic bronze serpent. The first set are likeable simply because the idea of inexorably following a purpose fits perfectly with machines, and the contrast between advanced machinery and the generally medieval setting creates a nice foil to their purpose of enforcing the proper order. The latter is a simpler case, where I like snakes, I like bronze, and I consider the merging of the two a good thing due to the aesthetic created.