If you want to learn standard chinese you need to go to Harbin. here they speak the most standard. furthermore the living cost is not as expensive as Beijing. Also here you really need to speak Mandarin compared to Beijing, Shanghai.

I don't think Hongkong, Taiwan or Singapore is the best choice to learn the stantard mardarin. Their mardarin are weird. In fact the great mass of cities in the North China are good choices, such as Beijing, Jinan and Zhengzhou. Although they speak with some local accent. You know any dialect can't be representative of the mardarin. But Beijing and Shanghai are still preferred. Beacause almost all the young people speak stantard mardarin. Beijing and Shanghai are both the most international and the best cities. There are a plenty of people can speak English or the others foreign language.

Here's an opinion from a Singaporean. If you absolutely must choose between Singapore and Hong Kong, I would probably recommend Singapore. But here's the catch, the standard of Mandarin in Singapore is not exactly top notch as we usually tend to use English at the workplace or in school. Even when we speak Mandarin, we tend mix in a lot of English words. To me, the best place to learn Mandarin outside mainland Chinese is definitely Taiwan, no question about that. The only problem is that they use traditional characters (it's the same for HK too). In terms of accent, Singaporean and Taiwanese Mandarin sound fairly similar (at least that's what I think).

Singapore ,Hong Kong ,Taiwan .You would meet many people who speak good English,so you can learn mandarin yet communicate easily.

between singapore ang HK, if you want to learn Mandarin , singapore is a better choice,but if you can go to mainland China ,northern cities are better than southerns'