It depends on a lot of things. If you're staying and working in Hong Kong, maybe Japanese is a better choice than the other languages. It's more practical & useful here in Hong Kong. But keep in mind that there're lots of people in Hong Kong who speaks Japanese nowadays, so if you choose this, you better work hard & be really good at it if you want to regard it as a bonus when looking for a job.
Internationally I think French is a better choice because it is the official language of many countries and numerous international organizations. It's very useful too when you go out and dine because you'll find that a lot of words on menus and wine lists are French.
You can have a look here for comparisons:
http://en.wikipedia.org/wiki/French_language
http://en.wikipedia.org/wiki/German_language
http://en.wikipedia.org/wiki/Italian_language
Actually I've learned French, German & Japanese before. I also had a few classes of Italian but the teacher was talking about movie stars and gossips all the time so I dropped out. Didn't want to waste my time on gossips :-) Can't remember anything in Italian except for the numbers 1 to 10 and "I'm a student." Anyway, my experience tells me that if your Chinese is not that good, learning Japanese will be quite difficult for you. I find Chinese difficult, Japanese even more difficult because my Chinese is not that good at all. With the Kanjis in Japanese, you have to remember by hard how to read them, just like in Chinese. If you don't know a word you can't read it out. You can't just "guess" the pronunciation. German is easier in this respect because you always read what you see. If you can read a word you can spell it too and vice versa. With French, there're lots of endings that you don't pronounce. You'll know as you learn the language.
One thing you need to know is that with French, German and Italian, you must learn each noun with its "gender". This is something you'll have to learn by hard. There're no fixed rules because as with all rules, there're always "exceptions". There's no such thing in Chinese or Japanese. If your English is ok, learning French or German shouldn't be a problem. You'll find a lot of "borrowed" words from the English language and vice versa. As a matter of fact, I've just picked up my French again. I think it's more widely used internationally.
The above is just my personal opinion. Hope this helps :-)