Age of Empires II. An old (8 years) RTS that keeps you coming back for more. I play on LAN all the time. Sure, it's got outdated graphics, but its gameplay is never-ending, and there is a TON of user made content, up to this day. I own two copies of the Mac version; you could run the PC version as well.
Or, if you like, you can try Age of Empires III, for better graphics, I feel the gameplay isn't as up to par as AoK, but it's still a very good game. I play it with people who aren't as serious about strategy games. Own 2 copies of the mac version as well.
The Age of Empires Series = The definition of what an RTS is. AoK is the reason we have the RTS genre today.