i will be taking a variety of focal lengths for various places. For a city that heavy populated i was thinking a prime , maybe 35 or 50. With a zoom option, i have 35-70, 16-35, 24-120. I thought perhaps someone here would have personal experience with that city.
35 or 50, if you wanna focus on the buildings the 35, the people the 50. No stinkin zoom will let you take pictures into the night like a prime. for 35mm I say 35 if you can only have one lens, 24 and 85 if you can have two.
I've been to Beijing roughly a million times, but you may not shoot like I shoot. I end up overwhelmingly using a normal lens---for me that's 50 mm, although some people really prefer 35. I can see places where a wide would be useful, but less so a long lens; unless you want to shoot skylines with a "stacking" effect, or something, any line of sight that would justify a long lens is usually blocked by a building, a taxi, or somebody selling DVDs. :-)
Get out early in the morning, while the city is just waking up, and walk around the residential neighbourhoods before the crowds really form. In my experience that's the best photographic time in Beijing.
San Diego, CA, USA
The lady of the house has to be a pretty swell sort of person to put up with the annoyance of a photographer.
-The Little Technical Library, _Developing, Printing, And Enlarging_