Although I don't like the main game too much by itself, the custom map community makes Warcraft III:Frozen Throne great.
Of course, you can play the RTS, which I am not very fond of, but I still bought the game.
This game has a whole other face, the custom maps, one of the most popular; DOTA (Defense of the Ancients) is one of the best. You control heroes and go down lanes fighting other heroes in a five vs. five brawl. The heroes all have different abilities used to kill each other, which makes this game a blast.
Another custom map type I like is the Tower Defense type, such as Element TD or Shopping Maul TD. In a TD, You build towers and upgrade them as they kill creeps. Some TD games have you maze the towers, and the creeps will walk through the maze of the towers to get to the end. Others, like Mauls, have the max amount of people in the game, all mazing together to kill off the massive creep waves that come. Mauls are usually only fun when everyone knows what they're doing, or if your with a bunch of friends. TDs also have the game setup where there will be lives that the player(s) will have, as players "leak" the creeps that get by there towers, the live counter will go down. Obviously, that isn't good.
But of course there are a whole slew of other custom games ranging from mazes, to many other stupid games but, you should definitely not miss out on this classic.