From what I've found, German is really only useful in Germany and French only in France. Spanish is useful in Portugal, Spain, half of south-America, mid-America and some African colonies. I think it's the most widely spoken language, even beating English and Chinese on that. Unless you're specifically planning on spending time in a country where they speak French or German, I'd go with Spanish or Chinese.
Keep in mind that Chinese is one of the hardest languages in the world, though, if not the hardest (here's an informative link that a pall who lives in China has given me). For as far as I know, you can spend four years studying the language and you'll still be speaking it at a child's level. While it might give you an edge, if you want to tap in to globalization then mastering Spanish in those four years seems like a better bet.
Some fixing is required:
German: http://wiki.answers.com/Q/In_what_countries_is_German_the_official_language
French: http://en.wikipedia.org/wiki/List_of_countries_where_French_is_an_official_language
Spanish: http://en.wikipedia.org/wiki/List_of_countries_where_Spanish_is_an_official_language
The most spoken languages in the world:
http://www.davidpbrown.co.uk/help/top-100-languages-by-population.html
Spanish is second after Chinese Mandarin.