Which English certification test is the most suitable for someone who only needs to have proof of his/her English ability?

I think topics included in TOEIC are more general than TOFEL iBT or IELTS.  Both TOEFL iBT and IELTS include some academic topics.  I also think one should take an international approved English certification test instead of a Taiwan approved English certification test like GEPT. 

Agree, yap..TOEIC are most relate to Business, and TOEFL iBT and IELTS are more challenge. However, if you are good at the English, doesn't matter which test you are get in.