Age of Empires is one of the absolute classics of the Real Time Strategy genre. It's fast-paced but has an excellent learning curve. For its time, it offered a diverse range of buildings, units, and tech-trees. Not to mention the differences between the cultures.
Though Age of Empires doesn't quite have the cachet of Warcraft or Starcraft, it holds up well even today as one of the best of the genre, offering lots of replayability at both the story and multiplayer levels. Additionally, the easy-to-use map builder tools allow you to create countless unique scenarios.
Age of Empires boasts fun and colorful sprites with detailed animations for all the various tasks each individual performs. Plus, there are a few neat surprises and hidden secret units (the iconic sports car with the rocket launcher, for example).
Age of Empires is a game you're sure to come back to time and again.