I'd suggest either Junky or Naked Lunch. Most of Burrough's books use a crazy cut & paste technique, that can make certain parts.. well, interesting to say the least, to get through. Not really hard to read, but making sense of it is a task sometimes.. Nova Express is crappy IMO. People will disagree about that.
His most popular book, and probably the one I enjoyed the most is Naked Lunch, so I'd go with that..