For most readers, start with Crying of Lot 49, or Vineland, or the short stories contained in Slow Learner.

But knowing a bit about you, Fox, I recommend Mason & Dixon. (Where else can you find two geographers fighting about how to draw a north arrow?) It is a challenging read, seeing how it is written in 1700's diction coupled with Pynchon's twisted syntax and constantly shifting point of view. But given that you enjoyed Stephenson's latest, I think you can slide right into this novel. Pynchon is one of the few novelists that makes me laugh out loud - repeatedly.

But you might enjoy Gravity's Rainbow as well. It has been more than twenty years since I read it but some bits are burned into my Brain: Byron the Bulb, Rocket Limericks, the philosophic difference between ionic and covelent bonds.....

Stay away from V until you get used to his style, unless you like page-and-a-half long sentences. They are brilliant sentences, but they'll make your eyes hurt looking for a period.