Personally, the language that's taught me the most to learn has been Haskell. It has a lot of very interesting ideas and a learning curve that plateaus after most other languages. There are several ideas that have trickled down from Haskell to other parts of the programming world and learning about them in the context Haskell is in my opinion better because you'll learn about them in a context where they fit in with the rest of the language very well instead of being late additions that offer an alternate way of doing things.

Coming from Java and JS, Haskell has a very different approach to a lot of things so you'll have to re-learn a lot before you get productive in it. This can be frustrating for some but you'll learn more if you get over that hump on the other hand.

Haskell doesn't see very much industry use and arguably isn't very well suited for industrial application (I haven't used it professionally so I don't know personally) so it might not directly help you land any new jobs but it is in my opinion it's a very good way to develop as a programmer.