Brainfuck, Whitespace, LOLCODE
While these languages where ment to be confusing and hard to read, under the hood they are as simple as you can get. They are essentially state machines. Now, obviously this does not mean that it is simple to write code with them, quite the opposite, but in theory they are as simple as you can get.
A more widely used language which I consider to be esoteric is COBOL. (hot take :D)
There is also a youtube channel who is dedicated to esoteric programming languages: youtube.com/@Truttle1
