OP, I have 2. 1) the city I started with and spent diamonds on like an idiot, and 2) the one where I play the game as it’s intended and also implement all the fun guides and strategies I’ve learned from the forum. It’s also my ‘testing’ city (I try things on it first before I implement to my #1 city which is further progressed.
They are two very different experiences, but I will say #2 is my favorite since it has my favorite guild.
I have one on every world, but I am only playing 7 of them currently...and those only half-heartedly. They have added too many features to the game, which makes it seem more like a job than a recreational activity. So I just dabble nowadays.
A couple of years ago it wasn't uncommon to see people with cities in most worlds, but as the game has added more and more time consuming features people have been dropping cities left and right. The majority of existing extra cities are almost certainly diamond farms, but I even notice those decreasing as it takes more effort to get wishing wells than it used to.