Haskell has definitely been the most rewarding programming language to learn for me personally. It's fairly unique in many ways, and really forces you to think

Haskell has definitely been the most rewarding programming language to learn for me personally. It's fairly unique in many ways, and really forces you to think in functional terms (unlike many other functional languages, which make it easy to drop back to OO or imperative style). However, Scala was the "gateway" language that helped me grok the core functional concepts after a long history of doing just OOP.