iippo: AI War: Fleet command is supposed to be great RTS, but I have never had the time to properly get into it. Its goddamn DEEP - which is great if you have the time :)
Immoli: I'll second this one. Pretty customizable too. There are different AI difficulties 1-9 (I believe 7+ is where the game is meant to be played) AND different AI types (
http://www.arcengames.com/mediawiki/index.php?title=AI_War_-_AI_Opponent_Types) looks like maybe over 2 dozen and they also have their own difficulty, easier, moderate, harder and then there is some technologist thing.
A lot of other settings to. You can mod the AI a bit more, forget how exactly. I know there is one where if you kill one AI (you fight two AIs at once) the other will send a huge wave to you.
You can also change threat level rules. The AI gets stronger throughout the game as you increase a threat meter which is increased by destroying things, decreased by destroying other things, and increases at set intervals. You can change by how much.
I think it is more of a 4X game though. All real time still. Has coop too, I think up to 8 people. No competitive because the AI works differently than the human players. It's not like other RTS where the player and computer use the same stuff.
http://www.arcengames.com/w/index.php/aiwar-features is the official site. Also available on steam which gives CD-keys that let work on that site's version (requires only offline cd-key activation).
I've hear a lot of goods things about Sins of a Solar Empire too. Personally didn't care for it, but a lot of people like it. Has more diplomatic shit if you care about that. The game allows multiplayer too, competitive and coop (by just agreeing to be allied to the person).
sounds interesting. And the System requirements are pretty low :D
I'm not looking to get into any Co-op, but I'll definitely check it out.