Age of Empires II is probably the height that the RTS genre will ever reach. I know there's an argument in there for Blizzard's RTS games and there's probably a couple people that would fight for Command and Conquer, but they are wrong, it's Age of Empires II.
I spent hours setting up LAN parties between my brothers and I, and we would have epic three hour long battles against the A.I. I'd hold the line with Teutonic knights, keeping their annoying small groups away from our early civilizations, while my brothers constructed siege equipment before laying waste to our enemies one by one.