We've been to Quanjude before and liked it. We went on a weekday and didn't see any of the crowds mentioned, and the duck was good, though can't say it was any better to the Peking Duck we've had, say, in Hong Kong.
Am interested in Bianyifang, which uses a closed oven as opposed to a hung oven used in Quanjude, but have heard mixed reviews, here and elsewhere.
I've also heard that Dadong is the best, irregardless of price, from many sources. Anybody that's been to all three that can comment?