I didn't really find anything amazing out of FFVI's opera scene, personally. *gets murdered by everyone*
Coolest moment? Hmm. Going by the word 'coolest' rather than 'best', I'd say that there are a few things you could go with that aren't major moments but were just cool. Riding your first chocobo, getting the Highwind, getting the Ragnarok (after using the garden for so long, the Ragnarok is amazing), and of course the introduction of the character that is Cid Highwind.
And while it was all virtual reality or whatever, I have to say that Crisis Core's scene with Sephiroth fighting that annoying poet guy was pretty cool. What else? Sephiroth disappearing into the flames. Rude being Rude. Balthier being Balthier. Shadow being Shadow. Discovering the backstory of Galuf is pretty awesome, too. President Shinra's supposed death in FFVII.