Gen IV marked a turning point for me; it totally revitalised my waning interest in the franchise. Gen III left a very sour aftertaste for me - Emerald was alright, but the lackluster design of Hoenn, coupled with the absolutely AWFUL music, really put me off the games for a while. It didn't really feel like the huge leap that G/S/C did to me - gone was day and night, the huge postgame content and, also, the hype. Pokemon wasn't very popular in my area when the third gen came along, and that was a factor for me at the time. Pokemon...wasn't really doing much. The anime had also gotten very stale, and I'd stopped watching it. It was becoming a thing of the past rather than a current interest.
The fourth gen...that was something else. The designs of the Pokemon really caught my eye first; the mascot Legendaries were essentially deities, and everything felt so much bigger in scope. The graphics had a huge upgrade, and I was of an age where I was impressed by those things. But what really made Diamond and Pearl special for me were the online capabilities. I had friends online who were into Pokemon, and we could FINALLY play together. It was fresh and exciting, and it felt like it linked more to the older games than Gen III did because of the large number of pre-evos and evos of original Pokemon. Plus there was the veritable pantheon of Legendaries. There was just so much about the games to love. Cynthia, the first female Champion. The return of many things left out in Gen III, as well as the physical/special split, adding new dimensions of strategy.
HG/SS were everything I could have ever hoped for in remakes, too. The overworld sprites, the Pokewalker, the GB Player, the beautiful redesign...I actually played a semi-translated mod a couple of days after the Japanese release because I couldn't wait. I imported SoulSilver from the US, then I bought two copies of HeartGold - one for me, one for my mother - and played it again immediately, even though I usually wait a few months before playing a Pokemon game a second time. I loved it.
Did I mention how great the anime was? I really took to Dawn as a secondary protagonist, and Ash actually developing some sense really appealed to me too; it was like he'd finally grown up a little bit and was more relatable again. The movies were also great; The Rise of Darkrai is my second favourite film, perhaps. I watched it avidly, I had people to discuss it with - and battle or trade with - and it felt like my childhood was getting a second chance. The hopes for another Gen IV were also what kept me going through Gen V, too.
I MIGHT start a new Platinum or HG file on the day. It's a long way off, so I don't have to consider it yet. But I do love Gen IV, more than any other Gen, so it's probable. Can't believe it's been a decade already...time for 3DS remakes, methinks. Make it happen, Game Freak. Skip Z and move on to this.