Sometimes, these discussions would move more broadly to his favorite physical features for all of his friends and then he was even more consistent: their smiles; he classified their smiles and had a favorite one for each person. It would be "the smile when he figures out a problem he has been working on for a long time," or "the smile when she looks at a person she really loves," and he would go on to describe why that was the favorite smile.
He always asked me about my favorite smiles, but I never knew, for anybody. I figured out my favorite two expressions for him after we went to a Yo-Yo Ma concert together. He somehow managed to have an ear-splitting grin simultaneous to a posture of total relaxation, mixed with complete, rapt attention.
My second favorite expression was that of skepticism each time I made any sort of statement, even something as mundane as "Asimov is a great author," and he would make me go on and explain my reasoning. Sometimes, I'll admit, this annoyed me but that expression alone was enough to deepen thought. When he made that face, he looked to the side and slightly up with a slightly scrunched smile.