It depends on what area you are looking for. There are too many institutions in Beijing, and only a few in each district are top-notch. Besides, it depends on the purpose of your school. There are also differences in the difficulty of mathematics training in various institutions. Haidian is almost the most difficult, and the exam atmosphere is the strongest. I wonder which district you are looking for. Let me recommend some for you.

Dongcheng: Shuntian Fu Xue (formerly Beijing Olympic School) is very famous. It is said that training points have also been set up in Haidian and Chaoyang.

Haidian: Learning and thinking, Gauss and Giant are both famous. Giant and Xuesisi are both national training institutions with good reputation.

Xicheng: It seems to be the old teaching association. I wonder if I have changed my name now.