A more natural translation would be to use which instead of what
I knew I would get this wrong because there are so many ways to render it, such as:
Which/what season is (the) best to visit/go to/travel to/in/ / for visiting/going to/travelling to/in Taiwan. Et cetera.
去 does not imply anything like traveling, visiting... though
Any season besides Summer. (Okay, summer is okay too but it's pretty damn hot and there are typhoons!)
What IS the answer???
"What is the best season for going to Taiwan?" marked wrong. Reported Sept 30, 2019
"What season is best" is proper English, and should be accepted as well