Let's see, fluency in German wouldn't take me too much more effort. I really just need to live there for a year or so. So I'll skip that one for the purpose of this thread.
So, I'd go with:
Japanese - it's awesome, and I'm an anime/manga/video game buff. While I took a year of Japanese, it was only a year and I forgot a lot of it. So I'd totally be down for the free fluency on this one.
Chinese - it's a tonal language and highly advantageous right now.
Basque - because it's a language isolate. Not much use in most places, but it would be epic to know~
And if I could cheat and have a fourth, I'd take one of the African languages that use a lot of clicks (can't remember what any of them are called off the top of my head) Exceedingly hard to learn if you don't grow up using it, so again, I'd take this one for sheer awesomeness.