Which ones do you already speak, OP?
If you don't already speak Spanish I think it's a must learn for any aspiring linguist whose native language is English. Easy pronounciation, near-perfectly phonetic orthography, straightforward grammar, and extremely widespread, giving you loads of travel and business opportunities. Plus it gives you access to a wealth of literature, films, and music from 'el mundo hipanohablante'. Not to mention the added bonus of getting Portuguese at about '80% off' so to speak, once you have mastered Spanish, unlocking the fantastic destination of Brazil if you choose to learn that variety. Italian will also come at 80% off if you speak French too, otherwise with just Spanish somewhere around 60%.