You can create something without being the most influential. Without sabbath there is no metal genre at least as we currently know it. That doesnt mean they meant the most to the genre in terms of how many bands they directly influenced or how they changed the perception of the genre.
Slayer were never afraid to be evil and satanic and push the boundaries while other bands like Metallica played it safe far too often.
Sabbath not only firmly established the genre as a whole, their various sounds set the stage for several different sub genres. The traditional heavy metal, doom metal and thrash metal sub genres all can be traced back to the influence of Black Sabbath. Slayer didn't do nearly as much, they were just a popular thrash band that attracted controversy at a time when metal as a whole was starting to push the whole evil image more and more anyway. "Being evil" is really not that important or significant compared to heavily influencing the development of the entire genre.
Slayer was a major influence on death metal bands that popped up after them. It was important.
In all honesty it's going to come down to what makes any of these bands the "greatest" in your own eyes though. Metallica, maiden, slayer, or sabbath are all good answers here depending on what exactly you feel has the most weight in an argument.
Going purely by influential or original youd have to round up an argument for Tool even (depends how loose your version of the metal genre goes as well)