If I want to go to Summer Palace from Badaling, which way is the best?

You can take bus no. 877 back to Deshengmen Station and the duration is about 1h. Then, take subway line 2 from the nearby Jishuitan Subway Station for one stop to Xizhimen, where you can transfer to line 4 to Beigongmen, which may take around 30min; use Exit D and the entrance of the Summer Palace is 250 yards to the west.
May help!