The original and, in my opinion, still the best. Age of Empires was one of the first RTS games that I got after buying a decent enough PC a few years ago. I still play it now. Newer versions have come along and other games have used the same ideas but the original is still the most enjoyable.
The graphics are dated by modern standards and the sound does a good enough job, but isn't anything to get excited about. It's the game-play that's the biggest asset though. There is enough content to keep gamers busy for a long time. Every game is different and so boredom very rarely becomes an issue. Games are easy to keep track of but the game itself has a perfect difficulty curve with levels gradually becoming trickier and requiring slight changes in tactics.
The Rise of Rome expansion adds to the life span of the whole thing with newer missions, campaigns and units.
As is normal, different play modes are available as well. The campaign allows you to choose a civilisation and work your way through loads of pre-set scenarios. Death-match gives you loads of money to buy an army as quickly as possible in an attempt to wipe out the enemy before they wipe you out. Skirmish allows you to select what settings you want in a one-off battle map. Scenario gives you pre-set conditions for winning. Multi-player is also there but for some bizarre reason I've never actually played it.
This game is simple in comparison to modern RTS games but is a lot more fun.
Brilliant.