SimTower + Streets Of Sim City
After cutting my teeth on Sim City 2000, I was in my local Cash Converters years ago and bought SimTower. It's awesome, and I still play it now. So simple, and you could loose hours playing it. The only thing that annoyed me with that was you were limited to the number of lifts you could have.
As for Streets Of Sim City, if you can find this on eBay BUY IT!!! You need Sim City 2000 to play it, and essentially what it allows you to do is drive around the city you've created. A bit like the way Sim City 4 Rush Hour works, but this is totally first person and you can't switch it off and go back in to Sim City. It is it's own beast.
Just a shame then, thinking of these three games I've mentioned, as well as the games mentioned in the comments, that the new Sim City will be a leap year away from the simplicity of the originals. I personally don't want a game that forces me to be online. I want a game like I fell in love with years ago. So really, I'll be stuck playing the original Sim City games instead of it's new incarnation.