Sinnoh, just for having a Bug-user in their rows
Well, seriously:
RBY Kanto (didn't really play FR/LG a lot):
Lorelei: easy, 'cept for Lapras, which likes to be annoying
Bruno: he fails. completely.
Agatha: Ghost Trainer? Poison Trainer. Quite challenging at times.
Lance: The Dragonairs and the Aerodactyl can be quite annoying. Rest is easy.
Blue / Green/ Gary/ w/e: Alakazam likes to sweep through entire teams, but the rest is mostly easy
GSC Johto:
Will: Easier one, but the STAB Psychics can hurt
Koga: Forry can explode, Crobat likes to abuse Double Team a lot, pretty hard one
Bruno: Weak, as always.
Karen: The Houndoom can be pretty good, but the rest fails.
Lance: Dragon Trainer? Flying Trainer. A lot harder than back in RBY, even though he lost like 15 levels.
Hoenn:
Sidney: Weak, since most of his Pokemon are pretty frail
Phoebe: Quite good, both Dusclops can cause problems
Glacia: The Walein is annoying, beating the rest is easy as pie
Drake: Pseudo Lance has a lot more variety in his team than the original, but he's still easier to beat, maybe due to Icebeam being easily accessable again
Steven: If he wouldn't have that Cradily, i'd say he's the easiest champion ever. But that things gets the boost from Ancient Power like 50% of the time, which makes it real hard to beat
Wallace: Easy, except for Ludicolo, which is anything but easy
Sinnoh:
Aaron: Lol at Beautifly and Dustox in DP but he got a lot stronger in Platinum
Bertha: Annyoing in DP with the Fissure Whiscash and that damn Sudowoodo, even better in Platinum, with the Gliscor
Flint: Lol at DP Flint, but in Platinum this guy became a real pain, damn Magmortar
Lucian: His Alakazam is even better than Blue's and the Bronzong... aarrgh...
Cynthia: Best champion ever, with a balanced team, good movepools, and being overleveled, she's really hard to beat
Ergo Sinnoh's Elite 4 is the hardest, and therefore the best Elite 4