What language should I learn?

I am looking for a challenge and have settled on learning a new language, new for me anyway. Now I need suggestions on which language to study. Please make the choices original (they do not need to be spoken languages hint hint or they can be hint hint) with a brief note of why your particular suggestion is worthy of a huge investment of time and/or why it is so fascinating that it can keep someone’s interest over the course of the time needed to become fluent.

Thank you for your help.