According to this post, the surface in an airport with the most germs per square unit is…the touch screen on the checkout device in the Departures area.
In a test, the self-checkin screens at an airport averaged 254,000 units of germ contamination. That was over 10x the number of germs on the armrest of a seat at a gate, or the button on a drinking fountain.
And the toilet seat? It had less than 200 units…only one thousandth the level on the check-in screen.