I'd have to point at 6502, Prolog, LISP / Scheme, and F#.
BASIC was the first language I learned. 6502 assembly was the second. 6502 was "hard" only because I was self-taught, and I was a kid.
(Learned 68000 assembly later, and my 6502 experience made me appreciate both the simplicity of the 6502, and the orthogonal expressiveness of the 68000.)
I learned Prolog while I was a linguistics major. It was very different from the procedural languages I learned. I had to do a lot of learning on how to use it. (Shout out to Dr. Kac, he's awesome!)
I learned LISP / Scheme as a computer science major, with a professor who didn't click for me. I wouldn't call them "hardest", but rather "least appreciated at the time" and I didn't like them much. I think I have better perspective these days to appreciate how powerful they are.
Haskell is pure FP, Scala is OO w/FP, F# is FP w/OO on the edges (the OO being necessary to fit into the .NET context).
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