I agree with everyone about how great Snow Crash and Diamond Age are, but my favourite of Stephenson's is Cryptonomicon (all about WW2/modern day cryptography). I like all three of these books enough that I'll re-read them every once in a while.

On the flip side, I just can't seem to finish his "Baroque Cycle" series - whereas Quicksilver started out strong, the last book (System of the World) just seems to keep dragging on and on. At this point, I couldn't care less about the characters and just want to get it over with and back on the bookshelf..