Just to let you know I will provide Wikipedia links to all of the incidents, which are actually very good and well written. What is below is only a small summary of what occurred.
Well, for the crash of a single aircraft it is Japan Airlines (JAL) Flight 123, out of the 524 people on board 520 died. The rear bulkhead was damaged some seven years previously in a tail-strike incident. It had not been repaired properly and on the fateful day the bulkhead gave way due to fatigue, blowing the vertical stabiliser off and causing the aircraft to plunge and soar for some 32 minutes. She finally crashed into a mountainside, with the huge loss of life being somewhat attributed to the remoteness of the location.
For the deadliest crash involving any number of airliners it is the Tenerife airport disaster which involved a Boeing 747-100 and 747-200 of Panam and KLM Airlines. 583 people died when the KLM collided at high speed with the Panam aircraft (which was still on the runway). The disaster was caused by poor weather, poor communication and the inadequate facilities of Tenerife Airport.
However, for the disaster which caused the most casualties due to incidents caused by airliners you need look no further than 9/11, over 3,000 deaths of the people on the aircraft, in the Twin Towers, on the ground in New York and in the Pentagon.
http://en.wikipedia.org/wiki/JAL_123 (Japan Airlines Flight 123)
http://en.wikipedia.org/wiki/9/11 (because of completeness, ten years on we all know what occurred).