Well, I'd say V. since it's his first novel and introduces some of the characters and themes appearing in GR. It's fairly dense and confusing, but it's not too hard to follow.
The Crying of Lot 49 would be a good starting place as well. It's only 150 pages and the prose is relatively simple (relatively speaking).
Gravity's Rainbow is the big one, a fucking MASTERPIECE. Seriously, words cannot showcase the scope and brilliance of this book. Yeah, it's long and complex and most people don't get more than 100 pages into it, but it's fucking worth it. #2 book of all time. I started with this one and, while I don't regret it, I almost wish I'd started with something simpler.