Contract Buildings that Take Up Space

Contracts are the most profitable way to use your city's space.  In general, the contracts that take the least amount of time will give you the most coins per hour.  There are some exceptions.

The downside to contracts is that they can expire.  After the contract completes, you have 1.5x contract duration to collect before it expires.

Example:  A contract takes 2 hours to complete, costs 5,000 coins, and rewards 10,000 coins.  You started it at 1:00pm and it finishes at 3:00pm.  You have until ~6:00pm to collect before it expires.  If it expires, you only get back half the cost of the contract.  In this example, you will only get 2,500 coins instead of 10,000.

Population Limited Contract Buildings

These buildings are the ones that are unlocked every 500,000 population that you acquire.  In my opinion, the best population limited contract building is the Nuclear Research Center.  The Radioisotope Drugs contract yields 405 coins per hour per hour per square.  This is an even better rate than most of the unique contract buildings aside from 3 on my list below.  I don't like to constrict my real-life with short contracts, so the top 3 that take 3 minutes, 5 minutes, and 60 minutes respectively are not ideal for me.

Best 3:
  #1 Nuclear Research Center (Radioisotope Drugs contract) [405 coins/hour/square]
  #2 Household Goods Facility (Electric Kettles contract) [260 coins/hour/square]
  #3 Household Goods Store (Kitchenware contract) [242 coins/hour/square]

Unique Contract Buildings

While they cost assets to complete,  the unique contract buildings generally provide a higher coin rate per square than all of the infrastructure buildings.  The contracts mentioned below are attainable by using a reasonable amount of assets.  i.e. you can upgrade the SQ City or Historical Institute castles more, but you would be just wasting your assets.  Yes, I built these buildings and I do use these contracts.  But had I done my research first, I would have rather invested my assets into my airport or my trains first!

Best 4:
  #1 Emergency Services Center (Cleaning Up the Aftermath contract) [680 coins/hour/square]
  #2 Conrad Miami Hotel (Organizing a Charity Event contract) [666.67+ coins/hour/square]
  #3 Power Engineering Institute (Solar Cells Upgrade contract) [533.33 coins/hour/square]
  #4 Hydrology Institute (Turbulence Moduling contract) [500 coins/hour/square]

Worst 3:
  #1 Washington Monument (Staging Holiday Events contract) [55.56 coins/hour/square]
  #2 SQ City (Meeting of the SQ City Board of Directors contract) [80+ coins/hour/square]
  #3 Historical Institute (Stratigraphic Research contract) [112.5 coins/hour/square]