As far as personality goes, Lenora was probably the best God how the name Aloe was better..., as far as looks go, Skyla was prettiest and when it comes down to overall awesomeness... probably Iris was best. But tbh something about the gym leaders this generation was kind of boring, though I did love how they got involved in the story more than in past generations.
Hardest on first playthrough was probably the Striaton gym leaders, or Lenora since at first I didn't use Pansage and kind of stuck to Tepig/Pignite, and with the new experience system it was pretty annoying to grind (I usually grinded, but still!) The later gym leaders were waaay easier.
Sadly though it is true that Elesa and Skyla were easiest for me. Although everyone apparently had a challenge on Elesa, idk, I just levelled up my Pignite (which might've been an Emboar by then?) and I survived somehow, lol :(