Hard to pick a winner, since the most esoteric languages will be those that were only ever used by their creators, and there are many of those :-) But listing

Hard to pick a winner, since the most esoteric languages will be those that were only ever used by their creators, and there are many of those :-) But listing candidates is fun anyway. Here are a few: SNOBOL, an early string-based pattern-matching language K, a database query/programming language PROSE, a dynamic systems modeling language from the 1970s that introduced automatic differentiation Among languages most people would have heard of, though, my vote for the most esoteric goes to Prolog.