Well, if you are seeking to study the most widely spoken language (numbers of native speakers), then, according to the Summer Institute for Linguistics Ethnologue Survey from 1999, the top choice would be Chinese, followed by Spanish, English, Bengali, Hindi, Arabic, Portuguese, Russian, Japanese, German, and French.
However, practically, I would say Spanish would be your best bet as you will most probably come in contact with more Spanish-speaking people in the region of the world in which you live.
Personally (being a certified German Instructor) I would say take German as it is the basis for much engineering and technical writing, and it is a relatively easy language to learn with much history.
But whichever language you choose, it is wonderful that you are seeking to become bi-lingual. It is not only good for the synapses in your brain, it shows us to be part of the world community.