Based on my latest playthroughs of Reborn and Rejuvenation, which are Normal monotype in Reborn and Intense mode Psychic monotype in Rejuvenation:
Hardest ones
Reborn: Kiki and Luna. Kiki's Hitmonlee could easily wipe out my entire team after Unburden boosted Speed: all of my mons were slower and High Jump Kick is super strong. I managed to win due to Kiki doing pointless switches like switching out Machamp and sending in Lucario against my Pyroar and also because Slaking somehow survived Hitmonlee's HJK. I don't know how that happened, maybe it was a low damage roll or something. As for Luna, dealing with Honchkrow was much harder than I expected, then there was that Malamar with Contrary + Superpower. An honorable mention to Terra.
Rejuvenation: Narcissa and Crawli. I defeated Narcissa simply because the AI kept using Spectral Scream against my Mr.Mime; I changed it's ability to Soundproof just because of this battle (for the record Spectral Scream is a sound-based move, so it gets blocked by Soundproof). My Lunatone also tanked Gengar's Never-ending Nightmare like a champ, although I changed the field with Flash. In retrospect I think I could've done better against Crawli: my Oricorio had poor EVs and at the time didn't have Hurricane.
Easiest ones
Reborn: Adrienn and Aya. I was supposed to do a bit of grinding before challenging Adrienn (my team was approximately around lvl 68-75 iirc) but got lazy. To my surprise I defeated them (xyr?) rather easily. Defiant Braviary was a good pick for that battle. Aya, on the other hand, got absolutely crushed by Diggersby and Toxic Boost Zangoose with Facade.
Rejuvenation: Valarie, Marianette and Souta. Valarie and Marianette both got swept by A-Raichu. While I didn't defeat Souta on my first try, he was a bit easy for "easily the hardest Gym Leader so far": In my opinion a more defensive approach works wonders here: I set up dual screens with Meowstic, then tanked Braviary and Dragonite with Metagross and Lunatone respectively. Tailwind has ended by now, so A-Raichu and Alakazam went to town against the rest of his team.