UTF-8 is the best choice for the world, but not necessarily the most practical for Taiwan.
A fair number of people here use old browsers with the encoding hard-set to Big5, and may not be able to see utf-8 pages despite both a Content-Type header and meta tag. There are also issues with encoding conflicts on form inputs, file uploads, and other submitted content. Not to mention that Big5 includes codepoints for some Taiwan-specific characters that may not be translatable into utf-8.
All of the above are why the company I used to work at use Big5 for their chinese site.
So, it depends on whether the Chinese portion of your site is aimed at Taiwanese people, or outsiders. Also on how willing you are to exclude segments of potential visitors based on their obsolete software.