Kanto for me. The Kanto episodes were awesome! And there's one very good reason for that: the anime clearly started going downhill during the Johto episodes, I still enjoyed the Orange island episodes (even though I didn't see many of them D= ), but the Johto episodes weren't really that good anymore. The only reason I watched them was that I had got used to watching Pokémon, that is also the reason why I bother watching the Hoenn episodes.
Oh, there's another reason for why I like the Kanto episodes too. The Finnish voice actors during the Kanto episodes were actually *gasp* good, which is very rare for Finnish dubs. But they changed the voice actors somewhere during the Orange island episodes and now Ash sounds like a little girl, Jessie like she had just inhaled a tank full of helium and James like he would have swallowed a grape fruit -_-
There's one last reason: in the Kanto episodes Ash actually used Pokémon I like and didn't have uberly powerful Pokémon that always won. Sure Ash loses sometimes, but not nearly as many times as he did during the good ol' Kanto episodes. Why did he have to release Butterfree and Pidgeot and let prof.Oak take care of Muk? ;-; I have said it countless times before and I'll say it again: the reason for the anime going downhill is simply because of everyone losing their personality, first Ash randomly becomes a goody-two-shoes-ooh-look-at-me-I'll-do-all-kinds-of-dangerous-things-to-
help-a-random-person-I-have-only-known-for-five-minutes kind of guy, then Gary becomes a wimp, Brock's flirting is just annoying these days and I hear Misty isn't herself either anymore -.-
I'm waiting for the day when the Pokémon anime is restored to its former glory (as if that would ever happen) or when they start broadcasting the old Pokémon episodes again. (not that that would happen any time soon either, but... =/)