I liked Old Man Methuselah. It's not classic science-fiction or anything, rough around the edges, but Hubbard could spin a good yarn when he actually made the effort (nowhere is this more apparent than in Scientology, obviously).
If we're going for the sarcastic kind of answers, I'd have to answer History of Man. I used to be fascinated by that stuff, and laughed a great deal at Piltdown man, the clam, and so on.