RTS or Turn-based?

How much do you like combat vs. city building?

For RTS, the classics include Starcraft, Age of Empires, etc. Rise of Nations was also pretty fun. Rise of Legends was pretty cool, too. Cross cultural stuff with magic/fantasy elements built-in. These are all combat oriented. They have multi-player capability as well.

There are also the games like Caesar, Pharaoh, & Zeus and the Sims series, which are more world-building types with hardly any combat. These generally do not allow multi-player (CPZ don't, I'm not as familiar with the latest Sims games).

Games like Starcraft and Age of Empires are heavy on the micro-management (not sure if that's what you mean by "realistic" ). RoN and RoL are less so. That is, if you're playing multi-player; if you're just playing the computer, you can beat it without breaking a sweat.

For turn-based, you really can't beat the CIV series, IMHO. CIV IV just has so much you can do. It is turn-based so the game goes much slower than Caesar. It takes forever to build a theater in CIV (in a small city), for example. But the depth and possibilities are so much greater. So many more choices, such as technologies, units, culture, etc. And in the later expansions, they also feature the espionage option, which is pretty interesting as well, since you can spy on other countries.

There are also games from the Total War series (Rome, Medieval, etc.) which are more combat focused than CIV. I like these too, but I guess I just like CIV more.