Portal 2 expanded a three-hour experiment into a full game without losing an ounce of charm. The single-player campaign explored the ruins of Aperture Science's history, introducing new mechanics (gels, light bridges, excursion funnels) while deepening the lore through Cave Johnson's increasingly unhinged pre-recorded messages voiced brilliantly by J.K. Simmons. Wheatley, voiced by Stephen Merchant, was a perfect comedic foil to GLaDOS. But the co-op campaign was the real surprise, a completely separate set of puzzles designed for two players with four portals total, creating some of the most satisfying cooperative puzzle-solving in gaming.