What would be a good second language for an English speaker to learn?
I think that depends on why one wants to learn a second language.
1) You find the subject interesting
In this case, whichever language you find interesting is a good candidate, though obviously instructional materials and native speakers will be easier to find for major language than for more obscure languages.
2) You want a language that will be useful in your country
Then of course that will vary depending on what country you live in. For the United States, that would be Spanish, for obvious reasons. For Canada, that would be French, again for obvious reasons. (Yes, I know that there are relatively few French speakers in Southern Ontario or Western Canada, but even those areas have major corporations and federal government offices that would value being able to speak both English and French). For Britain, I suppose that could be French or German or a few other choices.
3) You have a specific purpose in mind
Is your ambition to become the world's greatest Starcraft player? Are you willing to spend years in Korea training with the best? Then maybe learning Korean is the right choice for you.
4) You want a deeper understanding of English
Here's the controversial one. IMO, if your goal is a deeper understanding of English, then there is no better language to study than French. English and French are so intertwined from centuries of interaction, and have such a huge vocabulary in common, that a study of French will deepen your understanding of English in a way that no other language will.