1st gen hands down. I've played games with all the starters and I love them all (Charizard the most though :3) and the pokemon just seem to have... personality? Yeah. I've played through gold, sapphire and emerald since red/blue and they're just not the same... I love gold a ton, but I liked the gameplay in emerald/sapphire well enough but I dunno... just not the same?
Plus I grew up on 1st gen. My brother bought me a gameboy and pokemon blue and I was hooked right away. Everything else has been fun but, in the same breath, lacking.
That's why I haven't played d/p yet. I've seen it played, and I'm still deciding whether or not I should get it, but I already hate the starters (bad sign).