Even though it could often be 'not fun', I would have to give the gold to Everquest.
Being deep in a dungeon & suddenly the mobs respawn quickly and everyone just basically finds an exit and starts running.
Dying was punishing (losing your 'hell level', all your primary gear, your learnt spells). No one stuck around and just waited to respawn and go fetch your things as a non corporeal entity.
Though what a feeling of accomplishment for what you earned & looted. Also the PvP wasn't just a mechanical meta-gaming construct, it was actually part of the setting / storyline.
Great memories!
Edit:
Honourable mention goes out to Baseball & Utopia on the Mattel Intellivision. Best controller ever for a baseball game.