Brock is a joke, even with Charmander. Just spam Growl and Smokescreen and your good to go. His signature move Bide, was basically 3 free turns to debuff his team. Giving Charmander Metal Claw in FR/LG made the fight easy mode.
Cheren was easy. Getting to level 15 before the fight was the real challenge.
Misty was hard for the high levels+good early game STAB move+Starmie.
Roxxie for having no SE options available. Magnemite was the only option for a reasonably easy fight.
Roxanne/Brawly for me. Nosepass is unusually resilient for an early rock type because it has decent SpDef and no 4x weakness and Brawly just shows how evil the boost move + healing items combination is. Hoenn leaders in general spam healing items more than in other generations, afaik.
Generation 1 - If you pick Charmander, first two are Hard Generation 2 - If you picked Chikorita, first two are Hard Generation 3 - Neither are hard because even if you pick torchic, you can train it to evolve and learn a fighting move for Roxanne Generation 4 - Same here as gen 3, your fire started can learn a fighting move, so The first rock type leader isnt a problem. Generation 5 - This one is tricky, because of the two different games. In B/W the first gym is ment to test you by using your starters weakness, so its a little chalenging, but you cn get past it with a lillipup without aprob, Lenora is a touph one, but you can get a fighting type early on to help you. As for B2/W2, if you started with Snivy its probably the hardest, sicne he is the weakest at the start and you also ave to deal with a poison gym (but pokemon you can catch can easily help you out (Riolu for Cherren and Magnemite for Poison Gym.)
Thats my take on the first two leaders of each reagion and there difficulty.