The one most commonly cited is screeched (nine letters). However, one ought to mention also scratched, scrounged, scrunched, stretched, and the plural nouns straights and strengths (all with nine letters).
The complete Oxford English Dictionary also indicates the existence of scraughed, scrinched, scritched, scrooched, sprainged, spreathed, throughed, and thrutched.
The Oxford English Dictionary also cites a single instance of the ten-letter word scraunched from the 1620 English translation of Don Quixote.
English is spoken as a First Language by more than 300 million people throughout the world and used as a Second Language by many millions more. One in five of the world's population speaks English with a good level of competence and within the next few years the number of people speaking English as a Second Language will exceed the number of Native Speakers.