Chinese here. Learn Mandarin. It is based on Northern dialects which are spoken by 80% of Chinese people. And it is the official dialect used in China. People are required to use Mandarin in Universities. People are also required to learn Mandarin in school.
Northern part of China is mainly flat field with little mountain, which makes the dialects similar. Some Northern dialects can even communicate without trouble. On the other hand, the Southern part of China has lots of hills and mountains making people move infrequently (in old days, before cars/trains/etc.) That's why most dialects are from Southern part. Cantonese is one of the typical Southern dialect. However, most Southern dialects cannot communicate with each other even if their speakers were live only miles away.
Northern part of China is mainly flat field with little mountain, which makes the dialects similar. Some Northern dialects can even communicate without trouble. On the other hand, the Southern part of China has lots of hills and mountains making people move infrequently (in old days, before cars/trains/etc.) That's why most dialects are from Southern part. Cantonese is one of the typical Southern dialect. However, most Southern dialects cannot communicate with each other even if their speakers were live only miles away.