Haskell. I tried three times so far, and stalled each time once I reached monad transformers. Scratch that: I consistently stalled at understanding monads. I understand the Maybe monad, but I couldn't wrap my head around the implementation of the State monad. Then I looked at monad transformers, and they made my eyes cross. (If Haskell's syntax was just a tiny bit less jarring to my Lisp-addled eyes, maybe I'd try more often than once a year...)