I haven't played reborn, and I have 2 playthroughs of rejuvenation so I'll just use those
Easiest: Angie. don't know how, but I wiped the floor with her on my first try.
Hardest: Souta. Sure I beat him first try, but goddam he sure as HELL didn't make it easy
Easiest Angie. Correct me if I'm wrong, but I think the terrain she uses in V12 adds an additional dark typing to her ice type attacks, so I got Inceneroar and a Houndoom to wall her attacks, a Medicham with Pure power and High Jump Kick and a Magneton to annihilate her Aloan Ninetales
Hardest: Flora, I didn't have a good flying-type with me so she was difficult to deal with the rain and all