I've never used it in production, but I have a deep love of Prolog, just because of how different it is from any other programming language I've used. As a programming paradigm, it found it as eye-opening as functional programming. What I found interesting is that you are operating on logical statements and pattern matching, which often means that the same "function" can be used for multiple different things. For example:

append([], List, List).

append([Head|Tail], List, [Head|Rest]) :- append(Tail, List, Rest).

Is a simple list append function: append(X, Y, Z) is a predicate that match if Z is the list of all elements of X, followed by all elements of Y. You can use it to concatenate lists, of course. But you can also use it to confirm that a particular list start with a particular sequence, or ends with a particular sequence! The idea that the same predicate can be used in multiple different ways is really fascinating to me. I have no idea to what extent that would scale in a large system, but it's very interesting