diplomacy because its not a real board game
Diplomacy: Because it’s not a real board game unless someone leaves in anger after 6 hours of “playing”.
People make fun of board-game-nerd board games (e.g. Agricola, Twilight Imperium, even some Catan expansions) because of the large amount of time spent learning the rules and setting the game up. I can confidently say that the ratio of setup:play time in Diplomacy is a very low 1:20! (30 mins setup/learning, 10 hours playing)