Personally, in order of appeal, it's like this for me:
Johto
Kanto
Hoenn
Isshu
Shinno
Kanto - I'm not going to lie, I loved the hell out of these games. For christmas, when i was 6, I got pokemon Yellow and a gameboy color, I thought it was the most epic christmas. I played this game all the time. I get nostalgic when looking at sprites from these games. Everything was so simple yet so epic for it's time.
Johto - Personal Favorite. I love all the pokemon from these games and everything about it. Main Reasons: You could catch all the legendaries that were in the generation (besides celebi). You could go back to Kanto. And Umbreon. Plus, the games were in color and featured an EXP bar! The Hero wore his hat backwards! And it featured Shinies!
Hoenn - Eh. It was alright. I liked some pokemon like Absol, Manectric, Blaziken... All the pokemon sounded like transformers to me. And there were tons of legendaries. The first 6 gym leaders were named after their type, haha... Norman... I liked the battle frontier though. (Please NO Remake Gamefreak!!)
Shinno - My WTF Game. Beat it in 6 hours and didn't like it too much. The male looked funny and Dawn looked like a bimbo. I didn't really like any of the pokemon except for Lucario and Gallade. There were tons of new evolutions for previous generations and I thought surely were would be no more generations...
Isshu - Until last year when this region was announced. I've played through most of it and it's pretty much okay. I don't like the fact that the bike moves extremely fast. I like the dowsing machine. Some of the pokemon look strange... Tabunne?... I like the continued use of Duo legendaries though. Shouldn't Gamefreak be done by now?...
Black FC: 5028 3793 0359 - Ryu