Im going to have to repeat what was said before, with DoW1, and World in Conflict being good choices to get ahold of a great game.
DoW1 was a real RTS, multiple buildings, tons of units out at once, no crazy hero crap, and if you get gold+Dark Crusade you have 7 races to choose from in a great game, add soulstorm and you have 9 races to chose from in an ok game.
World in Conflict is an rts, thats both well made, and that a campaign thats written and acted in better than a few movies that came out recently.
Now to add something new, Rise of Nations+Thrones and Patriots, great RTS and very fun to play, has tons of civilizations to choose from as you lead them from the stone age to the future.
and now I think im going to list all recomendable RTS i know of
Rise of Legends - Fantasy version of Rise of Nations, but slightly more restrictive in where you can build capitals to force confict. Great game with three distinct factions, and steampunk mech's for the humans.
Black and White 2- lacks multiplayer is the only thing I can say against Lionhead's game. The creature system is fun to teach and train, and the citizens are decently capable of handling their own economy. It also gives options other than war, such as impressing others into joining you. The god powers are also alot of fun, nothings more hysterical than lobbing a fireball at an attacking force.
Homeworld 2 - Ive never played Homeworld 1, but Homeworld 2 is a fun game with the same unique system as last game. And with its diverse ammount of units and counter units is fun to play.
Star Wars Empire at War - if you like RTS, and you like SW, then this game should belong yo you. It has all the great units from the movie, plus some from the expanded universe. you fight the battles on both land and space, and should you go imperial have the option to devistate planets with a Death Star.
Total War Series - TBS'ish overworld map + realtime battles makes this series exelent, and with a diverse ammount of games to chose from there should be a setting for everyone. (i dont recomend Empire, its a bit odd, though my opinion may change eventually)
Star Trek Armada - Great game, impossible to get ahold of but still needs fo be mentioned. has 4 races from Star Trek battling it out. those races are the Federation, Klingons, Romulans and the Borg. All of their iconic ships from the Next Generations Era are available.