Civilization IV is pretty great and it's replay value and addictiveness is unmatched, this is one of the few games that can really drive me to pull an all-nighter on a constant basis, once you start playing you can't stop! and games can last 3-6 hours easily.
Aside from that, I really liked Command & Conquer 3 for it's old-school factor, it feels really similar to the first game and it's just downright awesome in my opinion. Company of Heroes and World in Conflict are two awesome games in their own right but I'm not a big fan of RTS that don't have a nice base building, harvest-resource economy model, this is particularly a problem in World in Conflict (where it's more of a tactics game than strategy) but less so in Company of Heroes but both are pretty great games anyway. The Warhammer games are also recommendable if you like the CoH resource-model. Age of Empires III is also worth looking at, personally I love the entire series (AoE, Warcraft/Starcraft and Command & Conquer are my favorite RTS series ever) but there is a demo to try.
Finally, you can't go wrong with Sins of A Solar Empire, this reminds me of a real-time Master of Orion which was nothing short of an spectacular game. Well those are the extent of my recommendations unless of course you'd like to go old-school and download Command & Conquer Gold free from EA, just kidding, C&C Gold sucks a LOT for people who didn't play it back when it was first released, otherwise the nostalgia factor is priceless.