The one you're most excited about.  Motivation is a key factor when it comes to acquiring a second language.  Some of it will be an awful slog, and you'll be frustrated, especially at the beginning, until you get over the hump of the learning curve.  The more passionate you feel about your new language, the more of a chance you have to stick to it through that time period.

I'd recommend Chinese (cuz 1.4 billion+ speak it), or Spanish (which your profile says you're learning?). The reason for Spanish being it's part of the Latin fam, meaning once you know Spanish pretty well, Portuguese, French, Italian, and Romanian become easy. I'm not yet fluent in Spanish but can already understand bits of those four languages, and can read them pretty well. (Plus Spanish is 2nd biggest after Chinese)