I love city builders, from sim city on my friends fathers 286. (we used to print out the map and do planning "offline" on my living room floor)
Also played a lot of the Caesar series.
There has been a severe lack of good ones since sim city 2000 though.
The Tropico series have done a good job though, and the traffic simulation in Cities: Skylines is awesome.
For me, there are two key parts. Building and Simulation.
It is important you have freedom to design. Preferably also the environment.
I want hills and water and forests, and organically grow my city around them in some places, and bulldoze it down to a flat area in other places. Depends on the city I want to build.
I want to be able to build my roads and bridges any way I want.
BUT.
Simulation is very important. I shouldn't just be able to build the city in any way, there must be rules for what makes a well functioning city.
Simulation must be believable, and detailed
The whole challenge and fun for me is to build a city that is both beautiful and well functioning.
If the rules force me to build boring square cities its not fun.
If the rules are too lax, so I can build any stupid design with no risk of problems... also boring.
I want traffic simulated, I want economy and services to be simulated, and it must be challenging to meet all demands.
I like how in Tropico it is impossible to meet all demands, you have to chose which groups to please.
Random events like tornadoes and godzillas I've never been a fan of, but please give me gridlocks and blackouts because I designed my infrastructure poorly.
Or let me get wrecked by outlanders because I didn't build protection for my post apocalyptic city.
Things that makes sense, and can be planned to counter.
I can't counter a Godzilla... just clean up the mess