Languages are only useful if you have a use for them.
If you plan to travel extensively or live in a place with a language (Spanish in a lot of the United States certainly counts here), learn that language.
If you want economic opportunity, learn a language with a large developing market in whatever you're interested in getting into.
If you want to learn a language just because "it would be nice to know another language", you're going to have an awfully hard time finding any motivation to continue with it (I can pretty much guarantee that you've underestimated the time and effort involved in such an endeavour).
That said, you should also consider how easily you grasp grammar. Someone who has difficulty speaking metalinguistically or understanding grammatical systems of an unfamiliar language will probably want something with grammar closer to their native language. A lot of people have tremendous difficulty in these areas - you would not be alone. French, Spanish, Italian, and German are all pretty decent bets in such cases. Many Asian languages are radically different from English and many less-known European languages have extremely unfamiliar grammar like the case systems of Russian or many of the Scandinavian languages. If you're intested in Asian languages, you should consider writing systems: many Asian languages do not write using alphabets for publications intended for adults (it takes many years of devoted study to achieve competence in reading many of these languages). If grammar systems interest you, Slavic, Scandinavian, and Asian languages might be good. Otherwise, you'll want to think deeply before heading in that direction.
Also, you should be extremely wary of any instruction method that seeks to teach a language with no explicit instruction in grammar. These tend to sell themselves on the notion that children don't need to be taught grammar to learn a language. However, you are likely not a child who hasn't yet hit puberty (the so-called "critical period" for language acquisition). Adults, by and large, do considerably better with explicit instruction in grammar.
Finally, and this is just being nitpicky, you should completely ignore anyone telling you that a language is intrinsically "difficult" or "hard" compared to another language. The only metric here is similarity to your native language. While it may be quite hard for an English speaker to learn Hungarian (understatement, trust me), a speaker of Finnish would likely have a much easier time. Conversely, a Finnish speaker trying to learn Italian would likely have a harder time with it than an English speaker would.
If you plan to travel extensively or live in a place with a language (Spanish in a lot of the United States certainly counts here), learn that language.
If you want economic opportunity, learn a language with a large developing market in whatever you're interested in getting into.
If you want to learn a language just because "it would be nice to know another language", you're going to have an awfully hard time finding any motivation to continue with it (I can pretty much guarantee that you've underestimated the time and effort involved in such an endeavour).
That said, you should also consider how easily you grasp grammar. Someone who has difficulty speaking metalinguistically or understanding grammatical systems of an unfamiliar language will probably want something with grammar closer to their native language. A lot of people have tremendous difficulty in these areas - you would not be alone. French, Spanish, Italian, and German are all pretty decent bets in such cases. Many Asian languages are radically different from English and many less-known European languages have extremely unfamiliar grammar like the case systems of Russian or many of the Scandinavian languages. If you're intested in Asian languages, you should consider writing systems: many Asian languages do not write using alphabets for publications intended for adults (it takes many years of devoted study to achieve competence in reading many of these languages). If grammar systems interest you, Slavic, Scandinavian, and Asian languages might be good. Otherwise, you'll want to think deeply before heading in that direction.
Also, you should be extremely wary of any instruction method that seeks to teach a language with no explicit instruction in grammar. These tend to sell themselves on the notion that children don't need to be taught grammar to learn a language. However, you are likely not a child who hasn't yet hit puberty (the so-called "critical period" for language acquisition). Adults, by and large, do considerably better with explicit instruction in grammar.
Finally, and this is just being nitpicky, you should completely ignore anyone telling you that a language is intrinsically "difficult" or "hard" compared to another language. The only metric here is similarity to your native language. While it may be quite hard for an English speaker to learn Hungarian (understatement, trust me), a speaker of Finnish would likely have a much easier time. Conversely, a Finnish speaker trying to learn Italian would likely have a harder time with it than an English speaker would.
German is a good third.
