Just learn Mandarin. It's the official language of the PRC and as such it's spoken by most people. I think only old people in China only speak their own dialect, so unless you intend to spend all your time with old people you should just learn Mandarin.

To me, Mandarine (pu tong hua) is the most popular one and it originates from northern dialects. And Cantonese is also very popular especially for those doing International business. We also have Shanghainese, which also represents the quick economic development of Shanghai. Sichua Hua is also very popular. Taiwanese is hard for most people come from northern part of China.

Hey, Cherry! I notice u've such a wide range of language possibility... I just can speak eng. and fluent in Chinese. Hope it could be helpful. As to ur question, I think it's not so clear of itself. Taiwanise is just a dailect.