Unfortunately, the answer to your question has to be "it depends," and what it depends on is what you want to do with the new skill.
For most people throughout the world, the answer they'd probably give is English, actually. In this global village we live in, English has become the sort of leveling language that is used to help people communicate. I happen to live in Singapore right now, and there are four official languages: Mandarin Chinese, Tamil, Malay, and English. The national anthem is in Malay. The Chinese make up the vast majority of the population (about 75%). But EVERYBODY communicates in English if using their native language won't work.
The same kind of thing happens in Europe, where you have a whole polyglot of different languages. I remember walking up to somebody in Paris to ask directions, and I started in French, but he immediately says, "No French! German or English." Ok, English suited me!
For better or for worse (and I think it's often for the worse), lots and lots of countries and cultures seem to easily absorb American things and emulate them... Call it the "McDonaldification of the World" if you want. This has driven the use of English even more.
Some people have suggested Spanish, especially if you live in the United States, and that makes a lot of sense given that the Hispanic population there is making a greater and greater proportion of consumers, so it could represent a commercial advantage knowing Spanish.
On a different level, the idea of Mandarin Chinese has been suggested. While it's true that China is fast becoming a leading economic engine, keep in mind that they're doing so largely on the basis of them being manufacturers and exporters of products. That is to say that their market is purchasing economies (like the U.S.), and I think it'll be a long time before Americans become interested in a wholesale adoption of Chinese culture, as vast and ancient as it is, so knowing Mandarin Chinese may or may not be useful - again, depending on what you want to do with it. Another important point is that while Mandarin is the "official" Chinese dialect, it's far from the only one, and depending on where you go in China, Mandarin can be completely useless, even in a MAJOR economic hub (like Hong Kong, where Cantonese is king). If you pick Mandarin, be prepared to work pretty hard to acquire that one, especially if you're an English speaker without any introduction to Asian languages. Believe it or not, Japanese is a lot easier, since there are no "tones" that change word meaning, and there are two phonetic alphabets (where the symbols stand for sounds). In Mandarin, you'll immediately be dealing with tones and kanji, where the symbols stand for ideas - - and there are lots of them!
Personally, I think that the most useful language is one that you enjoy learning and using, and has some relationship to a culture that you like. I've been working on learning French for years because I happen to like France a lot.
But whenever I plan on visiting a foreign country, I ALWAYS make it a point to learn at least a few phrases (like "please," "thank you very much," and "nice to meet you") and use them whenever possible. Sometimes that turns out to be the most "useful" language, since even if I can only get down a few simple phrases, that kind of thing goes a long way in showing the people I meet that I care about and respect their culture, and you end up getting a lot more accepted for the simple act of trying to do it.
Whatever you pick, have fun with it!