What's one of the dirtiest things in a restaurant?

Some customers: Restaurants try not to get a bad reputation and so their imployees are supervised. This is generally true. So the dirtiest thing in most restaurants is the occasional customer that is filthy.

Many items: Some customers notoriously do not wash their hands often, even after using the bathroom. Bathroom faucets, soap dispensers, towel dispensers, door handles carry many germs. Use paper towel to turn off water & open door to leave. Condiment dispensers & menus are infrequently sanitized; so carry hand sanitizer with you to prevent transferring others' germs to your mouth/food. [more in comment].