Which portal to use for best comprehensive screening

[Tina Kapur]

Hello, Newbie here. Just have 1 unit /condo in chicago loop, that is now available for renting. Hence  I dont use a property manager/management company for this unit
Posted on Zillow , total of 6 applicants. 2 very interested parties.
What tool/website should I use to do a comprehensive background check? There are way too many websites.
tried

Innago but they seem to have only the application. In which they dont

even ask for SSN (I had to edit the application to supply that)?
Someone said ZILLOW is not recommended (not sure why)

Help much appreciated. Thanks!!