I'm gonna disagree with everyone here. Pokemon Diamond and Pearl were in my opinion the weakest entries in the series. I thought the games were slow, the music was dull, too many HMs, besides a lot of evolutions of past Pokémon, most of the new Pokémon in that generation had absolutely abysmal designs in my opinion, the characters were for the most part unmemorable or annoying.
Honestly, the only generation I can say I don't enjoy. Platinum was a step up, with challenges like the Distortion World, the Battle Frontier, legendaries, but it wasn't enough to make me like the Pokémon more, the region more, or the characters more. The 4th Generation was never my favorite, and ever since I first got Pokémon Pearl back in 2007, I never really liked the games.
As a kid, I always went back to Emerald as a kid. But it's not just nostalgia:
-When HGSS came out, I loved those games. I thought they were great, and I thought there was so much to do.
-BW were good, and had an excellent story. B2W2 were phenomenal and had an excellent story. Of course, I wasn't able to appreciate the story fully at that time in my life, but as I got older I appreciate it more and more.
-X and Y were okay, but they had the awesome Pokémon and beautiful region to make up for the lack of any interesting characters or good story.
-ORAS was good, but not great. All I can say about those.
-Then Sun and Moon are by far the best Pokémon games ever. Absolutely phenomenal.
If I had to guess nostalgias never been a big factor for me. I just don't like the 4th generation. I argue my points with friends at school often, who think Generation 4 is the best. Of course, that's their opinion, and there's nothing wrong with that, I just don't see what's good about the games. Never once in my life did I ever find these games very fun. Then again, I don't have any nostalgia geared towards these games. Ever since ORAS was announced, I've been waiting for a DP remake. It frustrates me that there is a generation of Pokémon I don't enjoy. I want to enjoy it, and I think a refreshed version of these games, with maybe some Sinnoh forms (yes, Alolan forms for Sinnoh) and a more difficult storyline then I'd be very happy to see those released.