as i said before, what is most useful depends on what you want to do with the language.
if you want to learn a language just for fun then you may as well go bold. pick something challenging that will take time to learn. korean, japanese and arabic are said to be challenging. or, you could pick something a bit unique like esperanto or latin.
if you want something actually useful then it depends on what you want to use the language for and where you want to travel and use the language.
if you are chinese or live in the far east languages helpful would be adjoining countries. this would be chinese, japanese, korean, hindi, russian or similar. if you are european this would be spanish, german, french, italian, russian or as i stated before an adjoining country you wanted to visit. if you lived in the west the languages to know are spanish or french.
or, you could pick something relating to a hobby. for instance if you watch quite a bit of anime or read manga picking up japanese would be fun to learn. or, lets say you like a foreign music style you could learn the language to understand the lyrics. this makes learning a language more fun instead of studious.