Definitely THE most significant imo, I'm pushing 60 and was off work that Tuesday, watched it all unfold after it broke on Radio 5, never left the tv until early hours of the next day, it was like watching the most far fetched disaster movie imaginable and it was played out live on our screens, we had CNN & (I think) Fox News via Sky subscription and they didn't hold anything back at the time, it seemed like war was breaking out as reports of the other aircraft crashes came in, didn't know how long it was going to continue. In my ignorance I assumed the plane's controls had been taken over remotely and they were just going to keep crashing until the authorities had got them all grounded, never occurred to me that it was an orchestrated multi-hijacking and that the perpetrators would willingly fly them into buildings, I thought it was only the Japanese kamikazes that ever went to those lengths.
I also watched the Yanks roll into Bagdad around 18mths later, felt quite euphoric at the time and marvelled at the locals twatting the statue of Saddam with their shoes when the marines pulled it over for them..
Never thought it would all still be rumbling on over 15 years later.