Any of Age of Empires 2, Mythology and AOE3 to me. I love the first game for pure nostalgic reasons, but it doesn't quite hold up to the later ones.
AOE2 is a straight up classic, streamlined but hard to master, and still easy to pick up and play.
Age of Mythology is just all out fun - god powers, myth units, heroes and the addition of favour make it unique, but it still plays as well as AOE2.
AOE3 gets quite a lot of flak from fans of the original, and to an extent rightly so, but if you look past the RPG/Home City element it's the deepest and most rewarding game in the series IMO. With both expansions there's 14 civs, including 6 completely unique ones. Just a shame that the last time I played online, there was one very overpowered civ (Japan).
And next month Age of Empires Online is coming out, which you might find worth checking out since it's free to play.