Age of Empires II: The Conquerors

I had my mother buy me the 4-games box for a birthday when I must have been 13 or so. I went through the games progressively, spending quite a bit of time with the Romans. But Kings and Conquerors are the ones where I burned hours, used LAN with my brother and then online gaming (in ISDN times...), and tried to improve my skills.

First, as with the others in the series, I had great fun in the campaigns, which were quite immersive, diverse and were this great way of getting interested in medieval history.

Now about the game itself. The first great thing is that you can play no less than 18 "races", each one giving you quite different cards and units to play with, and that are yet very balanced (their buildings also look different). Between the fast and skirmish ones, slow and buff ones, expensive cavalries or run-of-the-mill infantry / archers, and the key importance of siege (oh and boats), and with the different buildings, you can renew your combinations.

I personally played the Huns the most, followed by the Teutons and Turks.

AoE games typically play for 20-45 minutes, a comfortable and rather slow pace that at first favors crescendos and massing of units. Just that by itself was great fun, especially with 6-8 players on the map, because whatever your level, the balance of forces could take a while to break (what with the possibility to wall), the pressure on the first players attacked could be resisted by slower but stronger developers, and you could sometimes take refuge.

Later, you came to understand that reaching the castle age was probably the one key moment in a game (usually around the 20th minute, but can be faster, down to 14-17) as only good players would attempt a feudal-age kill. And of course that you can't throw away your units, and must carefully react to what your opponent is doing. As you begin to understand your mistakes, the game acquires a lot more tension, made of harassment, map control, defense and buying-time, key buildings and units.

edit: I've recently begun playing online again, as there's still an active community for it (on Voobly). I mainly play Nomad and Arabia. It can get competitive, and there's a lot to learn in terms of development builds, shifting units and combinations, attacking and defending tower rushes, using siege, being tactical etc. It's both rewarding and frustrating and addictive.