I wanted to go with Operation Mi'henbut everyone chose that one so here are some others:

FFVI: During the fight with Vargas, one Super Blizzard Tornado Fist and your whole party is gone except for.. Sabin XD

FFT:A Where you finally find yourself a piece of mythril(?) and Donad comes in takes it from you and breaks it.

FF:CC Not healing my friends... ultimate pwnge

Final Fantasy IX : Kuja casts Metteor, destroying Terra and when Kuja pwns Zidane, Freya and Vivi in Burmecia Temple and flies away on his badass Silver dragon!

FFVII - When Cloud hits Sephiroth with Omni

FFVIII- I know a lot of people say it but Quistis with the gatling gun a Dollet beach.

FFIX - Bahmut v Alexandra

FFX - Final battle with Sin i mean i got super hard to fight the dark aeons and monster areana, and then when uv got break damage and u go into sin its like over in 5 mins, after operation mi'hen too, serious ownage there.

In FF IX the scene with the airships fighting against the dragons was pretty sick. Or when Alexander protected Alexandria from getting demolished.

Alexandra VS Bahamut hands down. Serious ownage there

Could members please to try to put more thought into what they post.

The biggest scene of ownage I ever seen would have to be Bahamut getting lit up by Alexander in FFIX. And then the tree city with the tornado surrounding it(Cleyra I believe) getting lit up from Odin.