There are different ways of measuring 'largest', including 'number of flights', 'number of passengers handled', 'number of check-in desks', etc.
There doesn't seem to be much doubt, by whichever measure you choose, that London Heathrow is in 1st place. Equally, most people would put Amsterdam Schipol 4th. 2nd and 3rd places go to Paris Charles de Gaulle and Frankfurt. (Most sources quote CDG as taking 2nd place but others put Frankfurt ahead of it).