10World of Warcraft
World of Warcraft—lovingly referred to as WoW by its millions of players—could be higher on this list if there weren't so many different ways to enjoy the game. WoW isn't restrictive, so if high intensity isn't your thing then you're more than welcome to quest your way to the level
cap while working on your professions.
Throughout its lengthy history, WoW has given players some incredibly difficult challenges. During the "Vanilla" days in particular, the end-game raiding content was remarkably hard. For example, in a dungeon called Naxxramas, the Four Horsemen of the Apocalypse were merely sub-bosses. But they were sub-bosses that took professional guilds more than seven weeks to defeat. In comparison, later end-game content was dealt with in five days or less by the same guilds.
Working by yourself to complete a goal is one thing. Defeating end-game bosses requires weeks upon weeks of "gearing up" and to achieve ultimate victory the battle plan must be executed perfectly by anywhere from 10 to 40 people. One mistake from a key role player like a tank or healer and it's back to the drawing board while your guild leaders loses it into Ventrilo. Rage quit: likely.