I mean, the carthartic pre-final boss was fun (and frustrating) enough, and was probably harder due to that bleepin'
Acendia
The music ( https://www.youtube.com/watch?v=OLBTIUzPpEQ possibly the greatest final battle theme of all time, the first 2 minutes are glorious enough and the rest are the background themes for different events in the battle, eat your heart out One-Winged Angel) is awesome enough to listen to if you haven't seen the battle, but even when you're expecting it, when you finally hear that choir begin in the back ground as you see (don't unspoil unless you've beaten or want to completely ruin it for yourself, trust me, you don't)
Ouroboro's true form
space as your battleground
he starts destroying parallel versions of your planet with a single limb, just because you know, HE CAN
I currently have it on sleep just to wait till no one's home again to fully enjoy it. It's also probably the only battle in the game I don't keep on fast-forward, just to make it last longer.
Ok, done ranting now.
But seriously, haven't even finished the battle (NO SPOILERS PLEASE) but it's already my favorite of all time.