About two years ago at the WorldCon in Denver, I watched a panel in which one of the writers (I don't remember who) was asked which of his stories/novels was his favorite. His answer: the one he was working on now.
The audience laughed. I don't think he was joking.
For me, as a reader, the answer is disappointing. I have my favorite writers and my favorite work by those writers, and I want my favorite and their favorite to be the same. It doesn't matter whether the standout story was written last year or 10 years ago. The words on the page are timeless, eternal.
As a writer, I have a different viewpoint. If someone were to ask me that same question today, I would say: My favorite story, the one I get the most excited about, is the one I'm writing now. It is the best thing I have ever written, the story I'm most proud of. And if you had asked me the same thing two stories ago, or three, my answer would be the same. I guess, in a way, I have to believe that. If I don't, why invest all the time and effort and passion into putting the story on the page?