I'm not exactly unbiased, here; I absolutely loved Age of Empires.  The moment I got word that AoE2 was in the works, I knew I would buy it.  AoE had so many great aspects; real-world history, great graphics, intelligent  interface and build systems and excelent and varied resource gathering.  Everything you want in an RTS.  Their best achievement, though, was the  random map feature... endless replayability.