I suppose it was inevitable that APL would pop up in this thread.I have programmed about ten different high level languages. Forth and APL are the only two I found that needed a paradigm-shift in thinking to get "into the groove" with.
My roommate and I used to compete for shortest/most obscure/most hideously resource-consuming solutions to our APL problem sets. You could write a lovely little one liner that in the middle created, transposed, and otherwise mangled and eventually destroyed six-dimensional arrays to arrive at an answer that was a single number. But hey, 20 characters! Fortunately we were not graded on efficiency.
It's a ridiculous language but man oh man it was *fun* and mind-expanding, and that's really the value of learning a bunch of different programming languages in the first place.