Kanto: Misty was the hardest because I always started with charmander and she had a flipping STARMIE
Johto: They were all pretty easy...I always start with typhlosion so scyther's not a problem and miltank isn't too hard to beat since it's special defense is low...but I guess clair will have to be the hardest since you don't get a dragon type until after you beat her and she uses a kingdra with dragon pulse...but she's easy if you have feraligtr...
Hoenn: My starter was always sceptile and frankly, he made the gym leaders cake (even flannery since camerupt is part ground) but I guess I'll give the hardest to winona...her altaria actually managed to bring my sceptile down to red...
Sinnoh: let's see, my starter was was always either infernape or empoleon...roark's easy since cranidos is slow and has bad defense, gardenia is easy since you can sweep her team with a staravia, maylene was challenging cause of lucario, wake was a pushover if you have an empoleon, fantina was kind of hard actually but I beat her with a bibarel lol, volkner was a joke thanks to gabite, byron and candace are cake...so I guess I'll have to give it to fantina...
Unova: Definitely Clay and that excadrill...I start with serperior so it only neutrally affects excadrill and it's usually a servine at that point...first gym is obvious cake, Lenora is easy if you get a sawk, Burgh is a joke if you catch a darumaka before challenging him, Elesa can't do anything against a palpitoad, Skyla is a joke if you have a zebstrika, Brycen is a joke if you have a darmanitan, and the last gym is easy if you trained up an axew...but clay was challenging...after a few hone claws, it wrecks have my team with bulldoze or slash...I only managed to beat him due to servine's high defenses...