Johto *v* My favourite region ever ! Amazing towns, awesome Pokémon, each gym leader is interesting (while I don't really like Sinnoh gym leader...), and the "evil team plot" is great, the soundtrack is beautiful... I really liked 2nd gen games.
Sinnoh is, on the contrary, my least favourite region... I was disappointed by Sinnoh. I can't find a town that stay away from the others, there are even "useless" cities, if I can say, such as Floaroma or Solaceon, whereas in Johto we don't have towns like that... Then I'm not fond of the Pokémon from the 4G. And as I already said, the gym leaders are less interesting to my mind.