I had like 3 and a half playthroughs:
The half playthrough: Aya,the very first time I played reborn.I had no idea about the type chart and Aya's bulky pokemons schooled me.Heck,it was the first time I even saw a dragalgae.Then I rage quit hence the toughest for me.
The actual first playthrough(that I finished): With the help of RNG,I won with a lucky hyper voice against Aya.But the toughest gym leader for me was Charlotte.Charlotte basically plays like me which is hyper offensive except she is better at it.Then I again lost interest for the time being until I suddenly won with trick room + double rock slide combo.The easiest for me was Shelly.Just a simple fake out+fire pledge combo.
The second playthrough:This time I choose blaziken.It was first time using a blaziken and I was happy with it's performance.This time the easiest gym leader for me was Luna.Obviously because of speed boost,bulk up blaziken with HJk.The hardest for me was Ciel.Because noe of pokemons were that bulky.Hence 110 BP acrobatics tore through everything.
The third playthrough:This time I played on hardcore.But because of previous playthrough,it was much easier even though starting from Shade,gym leader had mega evolution.The easiest was Radomus because I sweeped him with a nasty plot mismagimus.The other leaders I sweeped were Noel,Luna and Adrienn.