The answer depends upon many factors, including but not limited to:
- your definition of "popular"
- your definition of "helpful"
To clarify:
Do you mean "popular" as in "lots of people in the world speak it"? ...or as in "popular on college campuses"? ...or something else entirely?
Do you mean "helpful" as in "will win me a job", and if so, what industry do you want to work in? ...or as in "useful for travel"? ...or something else entirely?
Another key factor is your personal language background and raw language-learning ability. Do you have any previous foreign language experience? Do you have any friends or family who know a foreign language?
One of the best sources to better understand language difficulty is the National Virtual Translation Center: http://www.nvtc.gov/lotw/languageList.html ...this next link gives a language-difficulty index: http://www.nvtc.gov/lotw/months/november/learningExpectations.html
If you are up for a challenge, and wish to earn a good paycheck through the use of your chosen language, then you probably want one of the Cat-3 languages: Arabic, Chinese Cantonese, Chinese Mandarin, Japanese, or Korean. If you would prefer a language that is a little easier, then start examining the language family a bit more: German and Spanish (already mentioned) are popular options as they are relatively easy and valuable for US businesses.
However, ultimately is boils down to your personal interests. I had a friend who was very interested in Mongolia, so he became fluent in Mongolian. While he has not always used the language in his work, he has often had an opportunity to do contract-translation (such as via NVTC, mentioned above) and keep his skills honed.
...oh, if you want a government job, Arabic and Chinese are gold.