Star Wars Galactic Battlegrounds, and its expansion pack, Clone Campaigns.
It's basically the same as Age of Empires II (which was also a good game, though not as good as the third, in my opinion), but better, and more...y'know...Star-Warsy. First of all, it fixed the major balance issues of AoE II, where a few Janissaries could take down pretty much anything. Each civilization is distinct and unique, not just in graphics (although that was a major step forwards), but also in abilities. For example: the Galactic Empire has fantastic mechs (AT-ATs and the like), but their airships are like throwing paper airplanes, whereas the Royal Naboo has fantastic airships but troopers who might as well be holding empty water pistols.
The second reason I think it's better than AoE 2, or even AoE 3, is the extra dimensions added by the Star Wars universe. Spaceships that can only be attacked by anti-air units and buildings, Jedi and Sith who can convert enemy units (as well as having awesome attacks), mighty assault mechs that crush all resistance...it's just so much MORE than the historical games, where it's basically people on land vs. other people and land and occasionally some boats.
You can still buy it used, but I must say, it's a real pain in the fundament to try to get it to work on anything higher than Windows XP. Still an excellent game, though.