I think I'm going to choose End of Eternity as my next book, I've not read it in decades... Gods Themselves is a cool one as well, and a nice commentary on the workings of science. It also has his only alien race [as opposed to the collective intelligences of Gaia and Erythro] and a pretty neat one I think. It might have been interesting if he'd done more with aliens, but even with his output there wasn't time for more than humans and robots!
There are many other alien races in Asimov, though admittedly only in his short stories. The excellent "Blind Alley" and the awful ones about the aliens on... Jupiter I think spring to mind.