As much of Asimov as I've read over the years, it's rather hard to pick a single favorite among the short stories, because they cover such a range, from the hard hard sf to the almost lyrical and poignant. Certainly one of my favorites would have to be "Eyes Do More than See"... very brief, but the sense of loss in that one never fails to hit me. I'd also have to recommend "The Ugly Little Boy", "The Bicentennial Man", "The Dead Past", "The Red Queens' Race" (even though it's an early story, before he was into full swing, it's a very nifty paradox tale)... There are just so many to choose from.... And that's not even touching on his humorous tales, or his mysteries, or ....