Best Language
After Spanish which is best language to learn?
5 Answers
The best language to learn is the one that you want to learn.
Julian is right. I don't know how good your English is, but the ability to speak Spanish and English covers communication in many countries. If it is the ability to communicate with more people then you may consider Chinese, Japenese Arabic and Russian but they all require learning a new alphabet. My brother learned Thai which is a tonal language and presents its own challenges. I see what Helado says about Romance languages - I started learning Spanish whilst still improving my French and found some words the same or similar but not a problem. However, many people have told me that to combine with Italian is very difficult as they are so similar. I love the cultural aspects listening to songs, reading books and poetry plus reading newspapers with a different viewpoint I have also enjoyed communicating when travelling. My brother in law is now fluent in Russian, but he works in a Russian speaking area. In the end to learn a language you need to want to whether you have a friend or partner who already speaks it, a wish to travel to an area, music or literature you want to explore, a work opportunity or you hear some words of a language and are captivated. Good luck, whatever level you reach, it is never wasted.
If by "best" you mean "most useful", that depends entirely on your circumstances. You haven't filled out your profile, so I don't know where you live, but if you are in the US, in some areas like Louisiana or close to the Canadian border, perhaps French would be handy to know. For business purposes, maybe Arabic or Chinese.
JC is correct, as usual, but you might find Romance languages easier to learn after Spanish, since they share common roots. A downside is that you might start mixing words up from related languages, so unless you are confident with your Spanish, it might actually be a good idea to learn a language from some other language family. You aren't as likely to find common words in Russian as in Portuguese, for example.
I agree with Julian. There's no limit to the number of languages you can learn (and you can always improve the languages you do know). 
Here are my personal favorites, in no order: -Japanese -Cantonese -German -Romanian -Russian/Ukrainian -Czech -Bulgarian -Hungarian -Spanish -French
