In any list of the 10 most influencial aircraft accidents, the crashes of 9/11 must be included. What happened that day to those 4 unfortunate airliners has had a vast influence on so much in the entire aviation industry. Massive overhauls in airport perimeter security, passenger screening procedures, advance notice of passenger lists, better security for ramp workers/vendors, lugagge screening for explosives etc.
Plus, the entire "rules of engagement" that use to guide cockpit crews in the event of a hijacking are been thrown out and replaced by a new set of guidelines. There can be no question that the events of 9/11/01 belong on any list of influential aircraft accidents.