I'm not gonna bother reading over the last couple posts but I did glance over them and going to drop in that Gen 4 is my favorite. Although there was a time I actually hated Sinnoh because I played it so much, I've managed to dig it again. Yeah, it was my "first" pokemon game, but I think it's pretty cool. You've got the eevee evolutions, the underground section, cynthia, etc. to name a few things I liked. Plus the sidegames were great too. HGSS, ranger series, PBR, etc.

As for my least favorite, it'd have to be Kalos. The region is neat and the games have cool features like megas, but honestly the story and gameplay was a snooze fest and I didn't really have a challenge beating any part of it.