Heinlein -- I remember discovering his books in the library as a teenager in junior high school. I read them all -- and I reread them every few years. Stranger in a Strange Land isn't my favorite, I think I like The Moon is a Harsh Mistress the best.
I recently reread all the Foundation books (Asimov) and was surprised to think they aren't really that good. Asimov had great plot ideas but was a pedestrian writer.