The word 'most germ-free' suggests something that is highly sterilized or free from germs. The antonyms for this phrase would include adjectives that convey the opposite meaning like 'filthy', 'dirty', 'contaminated', 'unclean', 'infected', 'sullied', 'polluted' and 'germ-laden'. These words describe an environment, object or even a person that is highly susceptible to causing or spreading disease. For instance, utensils and surfaces that are not cleaned regularly can accumulate dirt and bacteria, making them highly contaminated. Similarly, unhygienic practices like not washing hands or wearing a mask can make a person highly infectious.