What is the best investment for short term (3-6 months) for S$100-200K?

If your aim is to grow your money in such a short frame of time, your only legitimate hope is probably day trading. Even so, you should at least be profitable before you trade with a live account.

Otherwise, keep it in your bank or a cash management account. Whatever potential gains you can achieve by other means are probably not worth the risk of losing your capital.

With such a short time frame, resulting in a low risk tolerance, to almost guarantee capital, the only thing I can think of are cash management accounts. So I'd recommend Syfe Cash+, Endowus Cash Smart or Stashaway Simple.

Just my 2cents worth. You do you.