I will respectfully disagree with those that feel the stadium is universally loud, but color my claim with the fact that my experience in the various sections has been heavily influenced by the specific game situation.
The absolute loudest game I have ever experienced was the September, 2013 game vs the Niners. That was the game, and goal line stand, where we set the record for the loudest outdoor stadium in the world. I was in the Hawk's Nest, and I have permanent tinnitus from that game.
The next loudest was "The Tip" NFCCG vs the Niners. I was in the south end zone, and broke my glasses and dislocated my jaw on that one.
The south end zone was relatively quiet during the next NFCCG vs the Packers, but the game situation, and weather explain this. It was very loud whenever the Pack was on offense on our end of the field, and at the end of the game when Kearse caught the game winner in that end zone.
The first game of the season vs Denver, after we beat them in the Super Bowl was relatively quiet. I was sitting in the 300 level on the Northeast corner.
I have sat in the club seats, 200 level in a loss to the Cards.
300 level vs Giants last year. Pretty peaceful.
I have sat in multiple games in the two end zones, and multiple in other sections. I have to give the south end zone as the most reliably rowdy and loud.
I think it is universally accepted that the stadium has become more quiet and sedate over time. That has been my experience.
Your best bet is a rivalry type game against a team that either has plenty of rowdy local fans (Niners, Steelers), or against a team that doesn't travel well.
The amazing game last year, vs Giants, was significantly affected by the huge Giants fan base in attendance. They were very quiet, IMHO.