Judging languages and the opportunities they give by the number of speakers is rather misleading. For example, it is true that Chinese is the most spoken language in terms of the number of speakers. However, unless you plan to spend a lot of time in a country where it is one of the principal languages, you are quite unlikely to encounter monolingual chinese speakers. Moreover, if you do go to a Chinese-speaking country, you will have to deal with the fact that Chinese is a general name for a group of similar but not necessarily mutually comprehensible languages/dialects.
My point is that there is no a universal prescription for choosing n languages - it is very much dependent on the learner's personal situation: ability to travel, professional interests, family connections, circle of friends. If you live in North America, Spanish will open many doors, although it might be more helpful in California than in Quebec. Similarly, Russian is still a good vehicle for travelling around the Former Soviet Union, and the countries that used to belong to the Soviet bloc, as well as some far away communist satellites - like Cuba. In Baden-Württemberg, if you don't speak German, you may find French a lot more helpful than English... if people you deal with professionally or as friends are not too young.
Thus, I would suggest to base the choice on personal criteria:
- What is the second or third most spoken language in the region where you live? Does it has substantial number of speakers? (That is, if only .1% of population speak this language, it is not such a good choice, even if it is techncially the second most spoken language.)
- What countries are you most likely to travel for professional reasons or studying?
- What langauges are spoken among your friends and family, and will these friends or relatives willing to use their langauge with you?
- What country/culture interests you? E.g., what was the language in which your favorite books were written? Which region's political news interest you? It is important, if you are to take your language to the advanced level - in terms of keeping motivation and practicing.
- What langauge do you find cool and/or more fitting to your linguistic abilities? It is crucial for keeping motivation, which is probably the most valuable commodity when it comes to langauge learning.