Goosebumps. Each and every single one of them. In fact, today, I was going through stuff to put in a garage sale this weekend, and I found my entire collection: Every single Goosebump book he ever put out, also including Beast 1 & 2. I have them out in the garage for the sale, but im thinking I might hang on to them, that way I can share them with a future child of mine and properly turn them on to the world of horror, since Goosebumps did it for me.
If you wanna check out another good R.L. book, check out his newest one. The Sitter. Its an adult novel, and pretty much a whole nother side of him I'd never seen before. Its really a good read.