Whatever the little decibel machines say, I've been to a lot of big home games, and for my money, the loudest game I can ever recall was 2015 Oklahoma. I've been to a lot of loud games, but something about that Oklahoma game, it was great weather and the crowd showed up hungrier and noisier than I've ever seen. Those fans willed Tennessee the whole day through. That said, I was unable to attend Alabama last year, so someone could tell me that Bama game was louder and I'd have to shrug and say "well okay then."
The loudest single moment though belongs to Travis Stephens taking that screen pass to the house against UGA in 2001. I know we lost the game, I know it's the "Hobnail Boot," but the way the crowd realized Stephens had room to run, and their roar grew and grew as he ran it to the endzone, it was an eruption. It was unlike anything I've ever seen there. I've felt the upper deck sway before, but that was the only time I was actually concerned about the structural integrity of the stadium. Neyland shook like I'd never felt her shook before or since. If there was a decibel record to set, I've got to believe that play set it.