Rise Of Nations Gold is the ultimate RTS game.
This game tops them all in terms of depth, replayability and game modes. The Single Player campaigns and Conquer The World modes are enough to keep you occupied for a long time, and the skirmishes are customizable and varied enough, to also keep you entertained for years.
The game is accessible, has a great interface, is easy to follow and get into, and is by no means "RTS light." Yet it's choices and game modes create a game that appeals to both the hardcore RTS fanatic, as well as the more casual RTS fan.
Now that the final version that was released is over 3 years old, I've found that RON still holds it's own in terms of playbility and graphical appeal. I go back to RON over and over. Simply one of the finest PC games ever made. This Gold version is highly recommended over the original which you can still find in stores. The fact that you almost have to buy the Gold version online on Ebay or some other online source, makes no sense to me. The expansion (Throne & Patriots) was a stellar expansion that added a lot of cool features to the game, and shouldn't be missed.