I've never played Diplomacy, but I do remember an 11-hour session of Twilight Imperium back in my university days as one of my peak tabletop gaming experiences. Hours and hours of careful diplomacy and verbal agreements all coming apart in the final hour when three players vied for the final victory. I was never in contention, but I remain proud of not breaking my word in game to this day (and also helping cost the win for the player who screwed me over hardest).
One of my favourite board game memories is from a 3-player Twilight Imperium game where I managed to maneuver myself in such a way that the other players thought I wasn't a problem any more. Which allowed me to snatch the victory right from under their noses.
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