What have been your favorite or most memorable Advent of Code challenges from previous years?

Oh boy, how can i forget 2019 and the whole series of "intcode" challenges!

You had a growing specification of a sorta-pcode virtual computer, opcodes, etc. Your input was the titular intcode, a list of integers representing instructions and you had to execute that code and use that execution to solve the bigger challenge, e.g. play a simple one-sided Pong!