The final fight for sure. That includes all the battles from the first Ultimecia to the true form. Everything from the music to the atmosphere was perfect. I dont think any of the other final fights from any of the other FFs have been as good as this one. Theres just something great about it.
I also liked fighting all the bosses in Ultimecia's castle, Adel and that thing that looks like an electric shaver just before Seifer in Lunatic Pandora.