All languages are important, useful and beautiful. Which language you decide to learn depends on what you want to do with it. If you want to learn throat-singing in the steppes of Central Asia, I would recommend Tuvan; if you're interested in the Laryngeal Theory of Proto-Indo-European, then Hittite or Luwian would be a good bet; if you're keen to discover the secrets of traditional Andean medicine, then Kallawaya might be handy for you (but hurry before it dies out, in which case you'd have to resort to Quechua); have you converted to Eritrean Orthodox Christianity? why not learn Ge'ez, then, or at least Tigrinya? Of course, the examples I've given aren't the only reasons to learn any of these languages, you could learn them just for the sake of it if you want, but the point is: why limit yourself to just the same small handful of languages when there are thousands of fascinating languages spoken around the globe? Even in your own country, there are about 180 different languages spoken, some of them, such as Pirahã and Hixkaryana, so interesting that they even seem to call into question what linguists thought they understood about the nature of language -- they sound like important languages to me.

The most useful language for you to learn is one you will have regular contact with. French, German, and Chinese are good languages, but if you never consistently come into contact with speakers of these languages in the real world, what use are they?

if you travel, I would say Spanish or French as it opens up pretty much all the Americas and some of Africa and western europe to you. I think Mandarin Chinese though takes the cake based on numbers

Spanish, of course :b