For me it was apl. It wasn't so much the writing of the program, but trying to understand one that I had already written, or even worse, one that someone else had written. But that was many years ago shortly after I had learned my first language, FORTRAN. Since then I have picked up Pascal, BASIC, C, PL/M, tcl, perl, c++, Java, python and a few others, along with a number of scripting languages. A couple of them like c++ and java approached the difficulty of apl, but nothing else has been that difficult.
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