My rating is based on the fact that the airport is...

My rating is based on the fact that the airport is really well connected to the city, S-Bahn is right underneath the airport so that within 12 minutes you're at the very heart of Cologne and major transportation hub. From there, in my opinion Cologne is one of the best connected small airports I know.
The fact that the airport is quite small, overall check-in and security check goes fast without much hustle. There are two terminals with a few shops and supermarket which is open 24/7. Some might find the facilities a bit outdated but from logistics and convenience point of view this is a 5 star airport for me.