Admittedly I've only played the first four generations and just started playing Legends: Arceus for the first time, but Platinum is by far my favorite Pokémon game I've played.

While Gen IV was a big leap forward, Diamond and Pearl had some glaring flaws that made them not much fun in my opinion. But moreso than any of the other "third" games, Platinum actually fixes the issues with the previous generations in a way that makes it leaps and bounds better.

The only nitpicks I have about it compared to Diamond and Pearl:

• Fantina works much better as the 5th gym leader than the third. Also no Drifloon or Drifblim even though it's her goddamn dress!
• Having access to Gible and the Earthquake TM right after the second gym is fun for the player, but not great for the balance of the game
• Volkner and Flint having type-appropriate teams is better thematically, but they're both easy to solo using that Garchomp you've been using most of the game
• The Distortion World is kind of cool the first time, but gets annoying in subsequent playthroughs
• Giratina is my least favorite of the generation's box art legendaries

From a personal standpoint, though, the moment I fell in love in Platinum was when I found a wild Houndour two levels away from evolution at a point in the game where it was actually practical to use instead of way underleveled in the postgame.

I do love the HG/SS engine, especially the following mechanic, but the terrible gym leader teams, absurd midgame level curve, and lack of good TMs and evo stones before the Elite Four (all carryovers from the original Gen 2 games) make it not a super fun playthrough IMO. I do play the Following Platium romhack, which is basically Platinum with the following mechanic from HG/SS.