Most good franchises have been named already with C&C, Total War, Starcraft & DoW. But I figured I would mention Sins of a Solar Empire. A fantastic space simulator RTS with amazing features and games can last weeks! Make sure to buy the Trinity version though as that contains both mini expansions (Diplomacy & Entrenchment). Also the mod support for this game is awesome with development teams on all major SciFi franchises (Star Trek, Starwars, Babylon 5, Stargate & a couple more). If you are into stories however please be aware this game does not have a single player campaign. It's just skirmishes & online play.