The best year to visit the wall is in spring or autumn. But the winter is also Ok if you enjoy snow scenes. As a young and single traveler, I suggest you visit Mutianyu Great Wall for good view, less people and security concern. Aviod Jiankou, for it is very dangerous (more than you can imagine). Aviod Jinshanling or Simatai, though they are wild and challeging, you may get lost because you are alone. Badaling is also a choice but the newly repaired wall will make you feel dull.