I'm lumping in the whole StarCraft experience here. I mean, what a masterwork! This is one of the most finely tuned RTS games ever. It has a phenomenal single player campaign, excellent online multiplayer, and even spawned an entire separate genre of games thanks to Aeon of Strife (which became DotA for you youngsters). This is a game you can spend hours in and never get bored of, and it has just enough going on to be challenging without the gameplay getting too overcomplicated. It was a defining moment for strategy games, and may very well be the best work Blizzard has done and will ever do.