Bejing, shanghai etc. I think many cities in East Coast of China main land should be fine. But I think you should choose a good school & college or university.
Taiwan is also good I think. But I never been there.
I think you Mandarin is already surprisingly good! So I wonder if you could understand the following I put in Chinese:
其实,我觉得,台湾人有很多都要比中国大陆人讲普通话要好很多。从我了解的主流电视媒体(CCTV4)上看来,很多台湾的电视主持人都有另人吃惊的语言知识,时不时地使用一些在大陆媒体的电视上不常看到的中文词和短语。