Latin, by far. I already know Italian and French.
<mega-facepalm>
why not a language that, you know, people actually speak in real life? 
Maybe the knowledge of Latin would be helpful on his scholary path? Or maybe he just wants to have fun with the language. It's not like he's limited to learning just one foreign language.
I actually learn Esperanto now. You're probably going to "mega-facepalm" on me, but I needed a new hobby and I have a great time with it, so I don't care.
No, no, not at all. What could be better than spending hours and months of study to learn a language that not one person in a million can understand?
not everything you learn has to be ~useful~ 
True, but I'd settle for "not utterly useless". Especially if we're talking about something that takes as much time as learning a language. OK, taking an hour to learn a couple phrases for ordering a beer or cursing in Latin or Esperanto, cool and fine for $hits and grins. But to spend months to hold a conversation with, well, yourself in a mirror.....? There are a billion other more interesting useless things one could learn in that time.
Poll
