Age of Empires II is the best in the series!!!
The goods:
Very good counter-system
Very good balance
Good campaign
Easy to learn
Solid AI for people who are beginners
It's a game, but you can also learn some history
The bads:
AI not too competitive for good players
Some minor bugs
Pretty dated now
The counter system and balance is very well done. All the civilizations are different, but yet they are all enjoyable to play and no one has an unfair advantage over the others. The campaign is good and you can spend several hours on one campaign line, which is based on historical events.
This game is very easy to learn. It has just the right amount of complexity and simplicity so it is well suited for casual gamers. It has a solid AI to introduce you to the game, but does not pose much of a threat to good players.
Another good part is that you can learn something from it. You click on "history" and it shows all the civilizations and most features about medieval warfare and you can learn something out of it.
Yes, it has some minor bugs, and this game is pretty old, but this gets put as a classic. I recommend you to buy this.