1. Generation I: Best characters, best rival "smell ya later gramps," best starters (Charmander, Squirtle, Bulbasaure, and Pikachu), and I love the music. Best Gym Leaders, and I loved battling team rocket in Yellow.
2. Generation II: Great sequel to the originals, great starters, and a fun-to-beat, no class rival. And I loved battling Red at the end knowing that he was you in the previous generation.
3. Generation IV: I liked Sinnoh and the starters were decent, my favorite being Chimichar. And I liked the story and the distortion zone.
4. Generation III: The storyline wasn't very good imo nor were the characters. But that's just my opinion.
5. Generation V. I hate Unova, and the starters, and most of the new pokemon for that matter, are ugly. I think GameFreak is running out of ideas.