I'd learn Arabic. It's easily the best language in the world in my opinion. The way it's written, the way it's spoken, pretty much everything about it is amazing. Moreover, most of my relatives speak Arabic or English (or both) so learning Arabic would be really helpful for me.
Japanese comes second, but by far off, since I wouldn't ever use Japanese in a real world environment. I only watch anime (which are subtitled), so I don't have any real practical use for Japanese. I want to learn it because of its uniqueness compared with other languages. I also want to learn Japanese because I want to visit it in the near future, and it wouldn't be fun not knowing my way around. But I could learn Japanese easier than I could learn Arabic, which is why Japanese comes second.