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Gen 5 is the only set of games in the series that even come close to most other RPGs
Gen 6 was a clear signal they were never gonna try that hard again, and each game since has shown as much, with Sword and Shield possibly being the peak of laziness in the mainline games. Much of what I would scrutinize with those games can be applied to earlier games, so I've never tried to tell people gens 1-4 were that much better, because in reality the only difference is that the excuses changed from incompetence to lack of effort. I've been with this series since the beginning, so it's not nostalgia either.
Even gen 5 has some notable issues, it's not much deeper mechanically than anything else, and while it's one of the more "challenging" games in the series it still isn't much.
Approached purely as a set of single-player RPGs, 5 has some of the best balancing in its battle system. The pokemon are distributed in such a way that any powerful option takes work to obtain, evolve, or otherwise optimize, there's no gift pokemon that solve the game for you like in later gens, since even the free legendary happens immediately before a fight with its counterpart. If you know what you're doing, there's also not really any grind, and even if you have to, audino exists specifically to speed up that process.
People bemoan its linearity, but I genuinely have no idea what games you've been playing, the only one with significant nonlinear progression is Gen 1, Gen 2 if you count Kanto. Did that really make those games better? The levels of the gyms in gen 1 still effectively force either excessive grind or... a linear gym order. In gen 2's case, it's yet another thing that contributes to the abhorrent pacing and level curve of the game.
I'm not gonna talk about how good the story is because every unova fan leads with that and I wanted to talk about everything else.
If you think I am not holding gen 6 to the same standards, tell me why.