Zone of the Enders: The 2nd Runner (possible spoilers)

The entire climactic ending sequence, particularly the HUGE battle before you get into Aumaan.
It's you (Jehuty), the Vic Viper, and 40 allied LEVs versus about 1000 enemy mechs trying to prevent you from breaking into their base. Not only do you have to fight your way through all of them, but you have to save as many allied mechs as possible. They're pretty useless on their own, and between you and the Vic Viper you'd be taking out most of the enemies. But your ranking on the success of the mission is heavily influenced by how many LEVs survive, and you will get an automatic game over if they are all destroyed.

The whole thing is truly epic in the scale of things: 42 good guys vs 1000 bad guys. Kingdom Hearts 2 tried to emulate the experience of this stage, but it's nowhere near as exhilarating or challenging.

And the final battle vs Anubis, where you are stripped of your armour and all but your most basic of abilities to battle him. This 'Naked Jehuty' is the most powerful in the game, as it is the pure, unbridled power of Metatron allowed to shine through.

ZOE2 is one of my favourite games. If a ZOE3 were to be made for the PS3, that would be the thing that would make me buy a PS3 immediately.