The 23 prisoner problem is a personal favourite of mine, I always felt the solution was so clever.
https://io9.gizmodo.com/either-you-solve-this-riddle-or-you-die-1653822870
What brain teasers made you "aha!" when you got them?
https://io9.gizmodo.com/either-you-solve-this-riddle-or-you-die-1653822870
What brain teasers made you "aha!" when you got them?