[TalnSG]

Though I have ben dabbling at learning both Japanese and Korean, from a business standpoint I would opt for Mandarin. At the very least Cantonese, since that is your secondary Chinese dialect considering its use in Hong Kong.

Personally, I am more comfortable with Hangul than Nihongo because it is easier to read and the phonetics are almost natural for me, but that isn't the case with everyone. I tend to remember more Japanese vocabulary though.

Given enough time and energy to devote to it, I would be studying all three. And "traditional chinese" rather than "simplified" should help with the kanji that is also used in the other two languages.