for me it is a toss of 3. War- The White Weapon from Type-0
Limit Break! From FFXIII-2
or Otherworld from FFX. And I know theres more rocky metal stuff from other ffs that are good so aye.
Also. If you wish you can choose a remix too. Just to widen the opinions here.
The opening scene of Blitzball blew me away. you really couldn't have matched a better song with that epic sequence of events when Sin strikes Zanarkand. I liked how they brought it back for the end battle too with Braska's Final Aeon, but the remixy kinda feel to it wasn't as captivating at the initial playing of the song when you fire up the game.
Pretty much any rock/metal-ish music by Takeharu Ishimoto (Crisis Core and Type-0 composer) :p
For examples see:
Crisis Core OST - SOLDIER battle
Crisis Core OST - Those Who Fight (remix of the FF7 boss theme)
Type-0 OST - Servants of the Crystal
Type-0 OST - The White Weapon
My favorite among them is probably The White Weapon