i guess depends on the team that u form.
my team is generally seen in my signature, but take out dragalge and galvantula and add swanna and linoone instead.
and it was like this kinda:
Julia, easy i guess, i don't remember anything.
Florinia, kinda easy, Linoone with field-boosted super-effective Pin Missile was MVP
Corey easy, i dont remember the battle
Shelly, i set the field on fire...
Shade was hard. His gengar lead was hitting very hard and field-boosted thunderbolt was preventing my slow Pangoro to do its job. I had to stall it until it depleted his PP so that i can sweep with Pangoro. (I just don't like scrafty)
Kiki, too easy, Swanna with Keen Eye was immune to field-accuracy drops and was hitting super-effectively
Aya kinda easy, too bad for her wasteland field does not weaken Dig
Serra easy. Butterfree had finally learned Quiver Dance... Noel was problematic for me solely because his Flame Orb Swellow was outspeeding and hitting everything too hard, while i had no counter for it. That, and i also didnt have a move to set the field on fire.
Radomus very easy for some reason, even though he had strong mons
Luna ... Earthquake to turn the field into a cave ---> Quiver Dance ---> Echo.. echo .. echo ..... . ..
Samson, i think i struggled some. My Swanna had become very weak cuz bad IVs and nature, and EVs all over the place.
Charlotte was the hardest for me, i had no water type, Sandslash was not resisting fire, so he could not set up Swords Dance + Sandstorm and still be alive. Until i somehow managed it. I think i used some revives..
Terra was swept by Victreebel...
Ciel was hard. Archeops Noivern and Gliscor were a nightmare...
TL;DR
Easiest something between Luna and Radomus,
Hardest Charlotte...
Still, the only leaders beaten at 1st try were Kiki and Radomus. So my "easy" and "hard" are through my own point of view.
If it was a nuzlocke it would probably end at florinia.