It might not be the most mind-bending, but it has been by far the hardest for me to learn (I thought HaskellHaskell and assemblyassembly language were easier!)
That is the set of HDLHDL languages, notably, VHDL VHDL (and verilogVerilog to a lesser extent)
Getting over the fact that every "function" runs at the same time is incredibly difficult and you can not avoid the shift to parallelizing absolutely everything. Of course, this is only border-line a programming language though.