OH no. I got a ponyta with me. I was prepared. But i gotta say a though gym leader is Probably Claire (Or Clair). In a gen where ice was her ony weakness, she is a one lucky DoB
but there was the ice cave, which already had pokemon viable to match the dragons after only 5 or so level-ups. Although I know what you mean. She is definitely one of the toughest 8th/final gym leaders. *cough*datKingdra*cough*
Anyway, on topic: For me, the toughest one is probably a tie between Tate&Liza and Whitney. But I'm giving the ball to Whitney. She appears only as the 3rd gym leader, but already has a pokemon packed with a pretty solid, all-around, and
deadly moveset (Stomp, Attract, Rollout, and Milk Drink). Tate and Liza's case is pretty similar. The synergy between their pokemon just works so well, that if you fail to exploit their flaws and weaknesses early on, you're gonna be in serious trouble.
As for the easiest one, it's most definitely Erika. By the time you get to her, your pokemon would already be tough enough. It was almost like a side-trip just beating her. Burgh, the bug leader, at least had a Leavanny that could actually swoop magmar butt. Ramos had a Gogoat that could literally bulldoze your team. Viola, while coming in as a runner-up, had a Vivillon that had Infestation - which could still wreck those who started with a water or grass starter.
If I was to list up the best leaders according to their Gym #, then this is what it would look like:
1st Gym - Brock / Falkner
2nd Gym - Gardenia / Brawly
3rd Gym - Whitney
4th Gym - Flannery / Wake
5th Gym - Clay / Morty
6th Gym - Jasmine / Byron
7th Gym - Tate & Liza
8th Gym - Clair
I'd actually love to see them to pair up the Leaders that way. Like, first gym, you get into a double battle against Brock and Falkner. That's definitely gonna be an awesome first battle.