I'm quite satisfied with two matchups:
Lance vs Diantha - Looks like it's going to have mid animation from the previews, but still is the most unpredictable battle here, and has a nice synergy with his Diantha seems to have prepared to counter Lance's team, with Aurorus vs Dragonite and Gardevoir vs Hydreigon.
Cynthia vs Iris - Not only does the animation looks spectacular based on the previews, but it's an excellent rematch for a battle that happened wayyyy back in BW. Although Iris is definitely the underdog here, and the writers would probably not lose the opportunity to have Ash vs Cynthia.
While Leon vs Alain has potential on paper, but when we got confirmation that it's going to be half an episode I lost hope for this, they will probably just use Alain as fodder to show that Leon's Charizard is still pretty strong even without gimmicks. Doesn't help that it seems like it's from the same episode as Lance vs Diantha, which is looking like it will have mid animation with that slideshow Moon Blast from Gardevoir.
Ash vs Steven is... Alright Ig? Unpredictable to say the least? Steven is the champion with the least synergy to Ash, he has already interacted with all the others, but Steven never had anything too significant with Ash. At least Ash is now facing Pokemon that he never faced before in this one, like Cradily and Mega Metagross, but why Steven when any other champion would have been so much more interesting is beyond me.