It's extremly rare for a manga to stay good for over 200 chapters, let alone 300.
We are 666 chapters in Kingdom, and it's still amazing and that's quite the tour de force.
There are a few weak arcs, there are a few weak characters but overall, there's not much i'd remove or do in another way.
9/10 and honestly i could give it a 10... it's just that a 10 should be something special and while Kingdom is really amongst the very best, it's not unique enough. Too "shonen-like". Not that it's a shortcoming in and of itself.
I think i've read Kingdom from bottom to the end maybe 3 or 4 times. And it was at least 400 chapters each time. It's just that good.
edit: they tend to be forgotten, but the translation team is also a huge part of the enjoyment! Thank you for your hard (and free) work!