IMO there are three golden standouts amongst the modern city builder.

Tropico 3. Set up your own banana republic in sun-drenched lands. Features customisable dictators, catchy music, and humour which manages to be funny. Oh - and great gameplay. This is such a perfect lazy Sunday afternooon game. Get it with the add-on pack, Absolute Power, as it adds loads of goodies.

Anno 1404. A medieval city builder come trading game come colonisation sim. So big you can get lost in it! It's practically endless, and filled with layer after layer of things to tinker with. That said, if you don't want to delve deep you can float on the surface and aim for pfun cities instead of production powerhouses. Absolutely recommended; IMO it's the best in the genre at the moment. The expansion is not that important and is arguably detrimental. Anno 1404 had the Ubisoft DRM patrched out so if you buy the vanilla game and patch it you don't have any worries. If you apply the expansion the DRM is back.

Grand Ages: Rome. As the name implies, it's a Roman city builder. It's a bit different in that it gives you selectable bonuses when you complete maps so you can start future ones with boosts that fit your play style. It's unique in having a military aspect which doesn't feel crammed on and which does not stink. You can send units (think TW style multiple men in one unit for ease of management) out to conquer barbarian settlements and force them under your wing. It's kind of like a modern day souped up version of what Zeus did except you're in control instead of relying on a random number generator. The expansion for this one is nice if you feel like adding a bit more to the game.