D&D has always been a kind of "base camp" for me. I have books of all the editions except 3.0 (I was on a tabletop gaming hiatus back then). I have more books in some editions than others, but I enjoyed them all. I have a slew of GURPS books, Rolemaster, Space Master, Dangerous Journeys, and a bunch of other titles I can't think of off the top of my head.
Recently I got The Dresden Files RPG and I have to say the production values on it is just amazing. It's also pretty fun if you have a creative group. Lately I've been eyeing up Savage Worlds and Deadlands as interesting alternatives to D&D.