Age of Empires
Since it was handed out on CD-ROMs as soon as you walked into just about any LAN party it's a wonder anybody bought Age of Empires. You could even get it for free in cereal boxes. Yet it sold three million copies and began a series that continues being supported today. Age of Empires stood out from the real-time strategy pack thanks to a historical setting that made it more like a high-speed game of Civilization than Warcraft. Its AI wasn't much of a challenge, but that made single-player deathmatches against multiple enemies more viable and it was one of few RTS games in which turtling (concentrating only on defense) was a valid tactic. It was another game that came into its own in multiplayer, where alliances could be made and broken, and everybody was on the alert for priests chanting “wololo!” as they converted your troops.