City

From what I understand now (I play synthetics, no need for silly things like food) food is a global resource. This way, you can create one planet of food and another planet of cities/population. That and food stockpile grows over time and food cost is a one time cost for cities. I could be wrong. Maybe it still works the old way with the base game. In that case, it is a bit of a trade off but the more population you have, the better the raw production is.

Asteroids is good for production too.

 The fact that food is global resource doesn't change anything. You still need two farms in the same planet or another plant to support the city.

Later upgrades to the farms make them +2 or higher per tile. Additionally, you can use the Food Distribution improvement for a % bonus on an agriworld. Alternatively, those isolated tiles on low class planets are good to use for farm tiles as the lack of adjacency bonuses make them less viable for research centers or markets.

percentages to farming really help.  Make a farm planet, and a couple of farmer citizens.  Suddenly, you are in the city-building business.  Just make sure to have a couple tiles for happiness.

^^Yeah, that's what I do. Currently, improvement buildings are far too weak. I build almost no factories/labs/markets. I pick about 10% of my planets to be food planets. If I have a good food planet, like a class 12 breadbasket world, that planet can feed almost my entire empire. I spam nothing but farms, Kimberly's refuge, and I'll settle a bunch of farmer citizens on it. 

As for my other cities, I'll give them one or some of the starter buildings, like the core mine or space elevator, for example, but beyond that it's mostly cities and morale and colonial hospitals. I do build the major buildings, like financial capital/mfg cap/research cap. 

If you have to even question the power of cities you have no idea what you're missing. Focus on growing your population and your economy will dominate everyone else. 

I try to put one city on any planet producing; Manufacturing, Research, Wealth, or Influence all other planets are farms, large planets might get 2. With that said I am not very good at this game though.

Is anyone abit annoyed that cities cannot upgrade or be destroyed? I didn't think much of it until I annexed a planet to find it was nothing but cities. My food and approval dropped like a rock and I think my food went negative (did keep playing long enough to check), my only option was to destroy the planet figured I could recolonize it and use it but it turns out when you abandon a planet it somehow gets reduced to a 0 dead world.

Honestly I don't know how y'all are still playing this game since the AI is incapable of building cities. The game is boring when you have 10x the production and research of the AI because it is unable to grow its population. 

Edit - I just verified in 2.61 the AI is capable of building farms and cities. 

Maybe it shouldn't go to 0, but I really think a planet should go to a lower level than it was before you destroyed it. That seems logical, after all, you have effectively blown a lot of things up - buildings, mines etc.