I wasn't sure whether to go with anime or game here so I went with game...
SPOILERS AHEAD!
For me, personally... it HAS to be Volo from Legends Arceus! The subtle teases at him having Cynthia's iconic team hinted at throughout with his Togepi and Gible were fun to pick up on, but they mostly just felt like little references as with many of the characters in that game... however, the plot twist of him being the insane villain obsessed with Arceus' power felt like the narrative had been dialed up to 11. As with the original Sinnoh games, the villains were Team Galactic and there was no such evil team established in Legends Arceus at that point, so it really begged the question of who was going to be the threat to the legendaries and, ultimately, the world.
Then the battle begins... and oh what a battle! The same Spiritomb lead as Cynthia, the ICONIC piano that sends chills down any Gen 4 player's spine as soon as they hear it, from having developed a crippling fear of the instrument in their childhood years, to the epic remix of the rest of her battle theme... then enter Giratina and its powerful theme too. My boy, the Pokemon equivalent of the devil, being summoned to battle alongside Volo making it a 7v6 unfair battle... but then, ONE MORE PLOT TWIST... Origin Giratina making it an 8v6 and you truly feel outmatched as you square off against this high level, high power team plus both forms of Giratina afterwards. Giratina is, by far, my favourite legendary and what an introduction it had in this game; being brought into the final battle and used as, essentially, a cheat to make the Cynthia 2.0 fight all the more terrifying, as well as unbalanced... and that's why I love this battle so much. It did everything right from a narrative standpoint, to the setup of Volo mirroring Cynthia's team, the music, the Giratina summon AND 2nd phase- everything combined just made it the perfect battle.
And, for bonus funsies... enjoy Alpharad's reaction to Volo's battle because of how relatable (and hilarious) it is:
https://www.youtube.com/embed/g1sNFH9QrPU?feature=oembed