It depends on your personal interests! Each of them has their own characteristics.

Badaling is the most famous one as well as the earlier one opens to the public, but it's too touristy & commercialized, only tourist go. Nothing funs, just a big name.

Juyongguan is a beautiful section in Beijing, it's considered as one of the eight best views in Beijing in the Qing dynasty (1644-1911), and it's a bit steeper than other sections. Cable car is not available at Juyongguan.

Mutianyu (highly recommend) is considered as the most beautiful part in Beijing, one reason is that Mutianyu lies in a beautiful mountain valley with orchards & pines, especially on the mountain side, unlikely the other sections, tons of trees make Mutianyu to enjoy in a pretty view all year round. Another reason is Mutianyu locates in Huairou district (one of 18 district of Beijing) where offers the best air quality in Beijing, so, hiking on Mutianyu making you taste the best air in Beijing.

Simatai, Jinshanling & Simatai locate at the border of Beijing, approximately 2-3 hours driving distance. All of them maintain the original appearance of the Great Wall, if you'd like to see the authentic wall with no restoration, you gotta go.