Really depends on where you live. Spanish is #1 here. You could challenge yourself and choose to learn Finnish, Japanese, Mandarin, or Cantonese. Russian is good because you'll be able to read all the hacking forums that don't get raided by the feds, and also good if you ever want an intel job.
Yeah, depending on how old you are and how much time you have Id either hit it out of the park with Arabic or Chinese or take the safe way with Spanish!