Best foreign language to learn? (for an American)

From the way you described your options it seems like already want to do Japanese.

First off, ditch Spanish. I doubt it'll help your career out, like, ever.

German's a solid language that a lot of people in the sciences talk. Lots of terminology in physics actually draw their names from German, so you could see learning some of it like taking Latin as an English major. And Germany/Switzerland/Europe isn't that bad of a place to spend a year or two at to do some work.

Chinese is a possibility, though I don't think it would be really useful as a physicist. If you were an engineer interested in supervising large projects, then you'd be pretty desirable for American companies that are subcontracted by the Chinese government, but since you're not, it doesn't really matter.

Japanese would be useful, but you have to think about how much time you'll actually spend learning this language. One semester of Japanese probably won't get you as far as one semester of German due to the larger gap in languages between English and Japanese from English and German.

Also, beware of Intro Japanese classes. They're usually packed with animu fags and kids that learned Japanese while growing up and are looking for an easy A. Totally going to kill any sort of fair grading scheme the professor can come up with.

Jam it back in, in the dark.