I'm torn between the battle against Blake on Ametrine Mountain and the battle against Solaris in the Fiore(?) Mansion.
The former has an amazing setting: On a frost-capped mountain, hail crashing down while you are up against one of the masters of Ice. Even though I don't like fighting Ice-types myself, them being my favorite type, I did not mind it simply because by that point I just wanted to bring Blake down. That, and, for once, on my Ice-Mono run, having battles before him where I had all the advantages and not my opponents.
Solaris, meanwhile, is just an awesome battle. Finally, you get to have a proper battle against the Pseudo-leader of Team Meteor and he does not hold back. Then again, neither should you. And just bringing down his demonic Garchomp at the end of a long battle, one that nearly killed us - twice - and killed Kiki...