I just returned from 2 weeks in China last week, including 5 days in Beijing. I managed to have Beijing duck 4 times, including at Quanjude and Xiao Wang Fu. My favorite duck however was at Hua Jia Yi Yuan. It's not a duck place per se, but it was fantastic! All the other items we tried there we great too. Best part of it was that the total bill for dinner for 2 was only RMB 190 (~$25). It's located on Gui Jie (this is a nickname for the street, the real name is really long and I can't remember it right now).

Xiao Wang Fu came a close second. I was hoping to try Li Qun but did not get a chance to.