I think there couldn't be only one.

http://www.myengineer.cn/

http://www.bjfbc.com/

http://www.qjfy.com/

If you are looking for classes for kids, look up this website :
http://www.flampekin.net

This is French for kids who already speak at least some French, and most of them are French or French-speaking already. They have classes Saturday mornings at the French school in Sanlitun.

Alliance Francaise is best for Chinese adults who want to learn French. But they are good too.

And then, there are a lot of universities that teach French, and some private schools such as Allons-Y in Haidian.

But it is also good to get a private tutor: there are a number who can teach their language, French, in Beijing!