Age of Empires 2

Age of Empires 2 is pretty old game, but still it does offer a very good coop multiplayer.
The game offers direct play over internet, just by IP which is very good for coop games if you have a static IP address.

We had a 2 vs 6 hard AI game last night, here is how we do it.
 Map settings:

Maps: Fortress or Arena for faster and relatively easier games and Yucatan or Rivers for the hardest games (actually they can sometimes turn into nightmare).
Map size: Large is best although Arena sometimes is a way better on largest, as you might find yourself bordering with AIs wall-to-wall.
Mind that Map size also affects lag as well as the population limit.

The rest is mostly a matter of preference, but we mostly use those settings.






Races: The easiest ones are Koreans, Britons, Turks, Spanish, Mongols and Chinese as they all have good ranged units. Enemies can go with any race, although we always skip Britons as they are pretty annoying (you have to chase those archers the whole time).

Cooperative play - the game offers a unique feature - playing with two players controlling a single nation (you just have to set the same color when starting the game). This can be fun to play - you are best to assign duties for each player - the most common case is to have general and governor, where the general will be attacking and the governor will defend and manage the base and supply reinforcements. When some of the AIs are taken out the governor can also get out and attack.

AI - The AI actually does pretty well, especially with enough resources and relics. They can produce tons of units (2 vs 6 finishes with up to 4000 enemies killed). Always play on hard or insane of course. The two river maps are generally the hardest ones as the AI would also make tons of naval units, actually the AI often makes zounds of them which can turn the game into a really messy one :) Island maps are a bit easier as you don't have those land connections with the neighbors.

Finally, the game offers an excellent coop play, diversity, unique feature to control one nation with two players, low lag and stable play on internet. This is certainly one of the best RTS games for coop.
A battle report from the last night's game will follow soon, check it out :)