Hitchhiker's Guide to the Galaxy. Man, I think I've read the whole series twice. I also enjoyed The Salmon of Doubt. I had a chance to see Douglas Adams speak live at UCSB when I used to go there but they sold out right before I got there, and he died a few months later v_v
If you want fun (and kind of cheesy [then again what fantasy isn't =P], but still good) fantasy, try the Sword of Truth series from Terry Goodkind. If you want fantasy that you have to commit your life to in order to enjoy, but is almost worth it, try Robert Jordan's Wheel of Time series.