For me it was Sting's return at Final Resolution back in 2006.
It probably has a lot to do with the fact that, as you said, not many people saw the peak of TNA. A lot of people have only seen the more recent, IMO much worse, stuff, so they're not exactly going to be rushing to praise it. There is a lot of negativity on here, but unfortunately a fair amount is justified.Probably The Gathering turning on Raven in December of 2003 in the Cage Match, but there's also the Barbed Wire Massacre Match of Turning Point 2005, Hogan and Bischoff's TNA debuts, Anderson's TNA debut, AJ beating Jarrett for the title at Hard Justice 2005, Angle's debut, Raven's debut, the first show on SpikeTV.
The quality of TNA's shows in the weekly PPV era is so underrated because so few people were aware of them at the time. They went all out for each show and it came off like an Attitude Era episode of Raw every time.
I find it sad that only a small number of people are willing to offer a shred of positivity and post in a thread like this. If it was a 'Worst Moment in TNA History' thread then you'd all be out in force trying to bury TNA.