Just reiterating what most people said but...

FFV: When Galuf defeated X-Death after getting pwned liked crazy with all those various spells (he died after, but still..!)

FFVI: Celes stabbing Kefka before the world was reconstructed to his liking. I was like "oh snap!"

When Phunbaba beat up Terra the first time

FFVII: Diamond weapon getting shot through with that mako beam from Shinra

FFVIII: Quistis shooting down the mechanical spider; very awesome CGI ^^

Squall getting stabbed with the ice steak; I luv him, but it kind of turned me on

As well, Seifer slicing Odin; I just laughed out loud 'cause it was so shocking ^^

FFIX: Beatrix beating the party various times. Oh yeah, especially the first time

FFX: When Tidus got punched by Rikku at the beginning of the game. Ouch

FFV: Galuf fighting Ex Death with ZERO hp.

In IV, Cecil fighting his dark self.

XII - Walking through the Eastersand and finding the Wild Surian 'Cannabilizing' a poor Wolf for the first time. Lovely.
When the Leviathan blow up, taking Ghis and the entire 12th Fleet with it. That'll teach you from stealing my precious stone you cunt.

X - When Rikku knocks out Tidus at Baaj Temple near the start, after just helping out in a boss battle. Didn't see that one coming.
Operation Mi'hen. Sin gives a big two fingers (well fins) to the Crusaders and eradicates them all epically before swimming away reaking of glory, but only after showing the Al Bhed some luv too.
The Auron flashback showing him going after Yunalesca for revenge, only to get owned with a click of a finger.

IX - When Black Waltz 3 killed the Black Mages protecting Vivi on the Cargo Ship. Buuuurned.
Kuja take control over Bahamut and uses it against Bhran, ultimately killing her. Elephant woman going doooowwwwm.

VIII - Seifer putting Odin in his place yet only to get beat by an ungrateful Gilgamesh. He just got you the sword you've been looking for and you've blasted him away! pffft.

VII - Cloud Omnislash's Sephiroth at the end. Epic.
During the dating scene if you piss off your date so much they slap Cloud. Outch.

and finally because I'm getting bored.

I - When I was rushing and accidentally used Attack for White Mage instead of using Cure, only for it to knock of the last bit of the big bad meanie that is Chaos' HP. Mwhaha funny.

IX:
- Atomos destroying Lindblum.
- Odin destroying Cleyra.
- Kuja levelling Terra
X:
- Mi'Hen Battle
- Sin's attack on Zanarkand
- The Airship crippling Sin

those are *real* pwning sequences, people bitch slapping each other just doesn't quite cut it in FF terms.