Rise of Nations feels like a true successor to Age of Empires II. It's simple to get into, it's complex to master, and it features some cool ideas and mechanics for the genre: there's a diplomacy component, the attrition mechanic is pretty clever, there's espionage, and even the Scout component of the game makes it easy and fun to get into.
That said, the reason I look so fondly to this game, is because of the hours I spent playing with this really good friend of mine. Those were very fun times, and newer generations deserve to play a really solid, really fun, really well designed RTS.