Meh, I played Gen I and II back when they each came out and I enjoyed them, but even at that time I thought the games were overhyped as all hell. I actually remember having an argument with some friends back in primary school about Gen II's awful level curve and how Johto was nothing but an expansion pack for Kanto with the Johto Pokemon getting sidelined leading to most people's Gen II's teams featuring no Johto Pokemon other than the starters, Lugio/Ho-Oh and maybe Ampharos, but they wouldn't listen lol
I stopped playing the games along with virtually everyone else in my class when Gen III came out due to Pokemon being seen as too kiddy for high schoolers, but I came back in 2011 with the launch of Gen V and I fell in love with the series again. I went back to Gen IV and played Platinum, which I loved, and HeartGold, which was alright, but there was no nostalgia factor there for me with HeartGold, I didn't love it like I loved Platinum and Black, it was just an updated version of something that I had already played 10 years prior.
Even though I played Gen I and II first I wouldn't rank either of them as my favourite Gen, they would rank near the bottom if I'm honest. If I had to choose my favourite Gen it would have to be the one that reignited my love for Pokemon, Gen V. The heavily reduced reliance on HM's, infinite TM's, the ability to fight the Elite 4 in any order(bout damn time), the ability to use another repel without having to go back into your backpack (game changer which had me crying tears of joy), difficulty modes (though GF could have implemented them better), Gym leaders who were more involved and actually played a part in the story, a pair of games which actually HAD a story (lol), an amazing postgame in B2W2, some of the best music (dat Route 10) and finally, but most importantly, the best Pokegirls ever seen.
If there is one Gen that I could ever see myself getting nostalgic for it's Gen V, and I'm someone who many would expect to be a Genwunner lol. I would never have expected PokeAmerica to become my favourite region, but there it is.
Also, wasn't the "Gotta Catch 'Em All" tagline only ever used in the west? I'm pretty sure Japan had something different.