Which part of Great Wall is the most often to be visited?

Badaling Great Wall is welcomed by most travelers, because it is not far from downtown, around75 km.
And the wall of this section is grand, part of the wall was built on huge rock. And travelers may visit Ming Tombs on the way to Great Wall.  Beijingservice offers Badaling Great Wall tour routes, including both private and bus tour.

Another part of Great Wall is Mutianyu Great Wall,  which is located around 90 km northeast of Beijing city.  Mutianyu Great Wall tour is getting popular because it is not so crowded as Badaling Great Wall.  And it is possible to take Mutianyu Great Wall  one day tour. 

Jinshanling Great Wall is around 160 km to downtown Beijing, but this part of Great Wall is attracting more and more travelers because of its grand building and unique characters. Jinshanling Great Wall tour covers one day tour as well as package tour, which offer you a chance to stay overnight under Great Wall.

Gubeikou Great Wall is around 10 km northwest of Jinshanling Great Wall. Hiking tour of Gubeikou Great Wall to Jinshanling Great Wall is very popular.  The hiking tour needs 4 hours waling on Great Wall.  But Gubeikou Great Wall tour is not so popular because the wall is far from downtown Beijing, and this part of Great Wall did not rebuilt till now. You may visit Gubeikou Great Wall if you are interested in visiting the ruined real Great Wall.