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).