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Well, German might be best for learning the English language and other Germanic languages like the Nordic languages. It's by far the easiest with the most shared vocabulary (~500 hours until fluency). However, in terms of practicality, it's fairly lacking.
Russian is good for studying Slavic languages in general, as most are mutually understandable to an extent. (Don't think about living in Russia at the moment, though.) Russian would also be useful if you are in programming or want to go on Vkontakte, as it is majority Russian. It's the second longest to learn (~1000 hours to native-like fluency), but you can likely leave the complicated grammar for later.
Japanese is useful for Japanese forums and anime. It's also a good country compared to Germany and Russia. Japanese can also be used as a way to learn Chinese and Korean via shared vocab or terms. However, pronunciation is far different than English and the three to four writing systems used can be hard, especially Kanji with its 2,100+ characters. It takes the longest (~1,200 hours until native-like fluency).
I speak Russian fluently so I can't give much of a pointer there, but Japanese I've been trying to learn and it's been extremely hard. I've also heard German to be very hard from my friend that is taking German. Choose wisely, young linguist padawan.
