I'll raise you one better and say that most generations of Pokemon aren't that great. I've been a lifelong fan so I've played everything from GSC to SuMo and I have nostalgia for a lot of them, but the majority of main titles fall very, very flat.
If I had to pick a top 3 worst, it would be DPP, XY, then Sun and Moon/ORAS in order of least bad to most bad.
DPP is okay with me because it has some tough battles but it'll never match the Battle Tree in White 2.
People get butthurt when you say this stuff but I think we should just admit that a lot of us like the idea of Pokemon and the cute imaginary animals but the actual games leave a lot to be desired.
Off the top of my head, there's little replay value if you're not relying on a player-made challenge like Nuzlocke, they're extremely slow to get through (not just the beginning parts but the whole game), the writing is subpar and fails to make characters that you can care about (in fact a lot of the time they're just straight-up annoying), and they have been getting progressively easier.
They say Sword and Shield were supposed to be "disruptive" and take the series in a new direction but it looks really same old. And the starters are really ugly. But I'll probably buy it anyway... sigh
E: I also second the "Kingdom Hearts Bad" train