Clicker Heroes. It's an idle game that they won't escape unless they click mindlessly for 500+ hours. I'll also demand they get all trophies/achievements that the game has to offer. After that they'd wish they weren't my enemy.
But this game is free, so nothing to gift here. For a paid game, I would give a less frustrating game named Celeste (assist mode deactivated, also need to be completed 100%).