Just one? (Score 2, Interesting)

Numerous MUDs for introducing the basic concept. Numerous Arcade / Home Console Classics for further introducing the GUI version of cooperative and adversarial interaction (e.g. Mortal Combat, Pong, Mario Brothers, Gauntlet...) If I had to choose one, I guess it would be Wizard of Wor, both cooperative and adversarial, and one of the first. I really would select Ultima Online except for the fact that it is an MMORPG and that is against the article's stated goal. I would not choose it for its MMORPG state, but for the way groups had to work together and the fact that it was the first of that nature that went big. Even if it hadn't been an MMORPG, the basic concepts were there similar to games like Dungeon Master, Eye of the Beholder, Etc. that required teamwork beyond you and one other person. Sharing supplies, using skills that augmented and/or supplemented your ally's skills, etc.