What has everyone found to be the easiest way to find the owner of an apartment complex?

Find property record at the county tax appraisal district. It has owner's name and address. 

If this is a person, your search is done.

If this is an LLC or similar structure, look it up at corporationwiki.com or opencorporates.com. You may have to decipher the entire corporate structure that they set up but eventually you'll find a person behind it.

The solution varies by legal jurisdiction. In many places, this information is public and you can look it up on the county tax assessor's website. Some states and provinces have greater privacy and keep the information in a private database. Some places require you to pay a fee to get access to the data. In areas with greater privacy, a lawyer or title company who have a subscription to the title records can often look it up for a fee. What state are you looking for? 

find someone like a lender with access to costar

@Victor Menasce

Hi! I’m looking for Wisconsin, in Fitchburg.

@Kyle Hoppman  

You can use skip tracing tools. A guy from one of such companies actually reached out to me recently. Here's their info: 

skip tracing tool idiCORE  Juan Hasbun

@Alina Trigub

Thank you!

Look on the county GIS it will tell you who the owner is and their address.

@Kyle Hoppman

Knock on the doors until you find a friendly tenant who will tell you who owns it.

@Kyle Hoppman skip trace the owner. We use batch skip tracing.