I think it would have to be "Everything that Rises Must Converge". I just did not enjoy this story. It went on for too long without anything happening. Not enough twists and turns for me. Also the language was dry. So thats my least favorite!
Now for my favorite . . .
I would have to say either "The Veldt" or "Hills like White Elephants", its a toss up! I enjoyed "The Veldt" because 1. I enjoy Bradbury's writing and 2. because it was so interesting and far fetched! The story kept me interested the entire time and that is very important to me! I also enjoyed "Hills like White Elephants" because of the author. Hemmingway is wonderful and keeps the reader engaged the entire time. So I don't think I could pick an absolute favorite!