I really fought hard to not include this one. I tried to make a mental case for leaving it off, based on how overrated it was. It does introduce the airship, which opens a lot of stuff up, but the Opera itself doesn't really move the plot that much, the character it introduces (Setzer, the airship's owner) is a minor character, and the player doesn't really have that much to do in the actual scene.
Then I started remembering the scene. Here was one of the first times I'd been treated to something with the grandeur and majestic sweep of cinema. It spoke to what video games could do in a storytelling aspect, as well as showing that cutscenes didn't have to be simply "point a to point b" action fill-ins.
...and that fight with Ultros. It wasn't that much of a fight, and he was mostly a comic relief bad guy, anyway, but the time limit put a nice sense of ungency into everything, and the part where the gang falls to the stage and gets caught up in the fight is a lot of fun.
The more I thought about it, the higher the Opera scene rose on the list. I think right about here is a pretty decent spot for it. I had intended on leaving it off entirely, but found that it struck a lot more of a chord with me than I had originally realized. It's a great portion of a great game that deserves all the love it's gotten over the years.