Star Wars: Empire At War is also a good game in this genre. Not sure if I'm biased though, hehe.
However, I really like AoE III... Can't speak for the previous versions, haven't tried them, but I know they were really popular.
Something in particular that I like about AoE III (maybe all of the AoE games, dunno) is that the maps are random, and you can customize your strategies a little more with the home city feature. Most other RTS games that I've played seem to feel like a race when in multiplayer... See who can hit the resources first and which heroes can take out the most static spawn points before confronting the enemy. Of what I've played with AoE III, you have to be able to adapt a little bit more and exploration is necessary. There is a general formula to the situations, but it still feels fresh even when you play it a bit.
I also like the setting, which is not as common in games like this.
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Anyway, I really do want to help with this faction. I'm not that great, haven't played in a while, but if we're planning on promoting KoA I guess I'll have a reason to get better.
Oh, and if anyone is having trouble running AoE in a way that is playable, try turning down the shader and shadow settings. For me at least, both had a big effect on my framerate.