I gave 8 to Cage of Eden, so I think the option I chose is accurate.

It's usually between a 7-8 for me. A bad ending means I wont bother to rewatch/reread again basically. Like GoT. I was tempted to get the series on Blu-ray but I'm not wasting my time rewatching it when it ended like that.
Sure the journey was great but the ending has to be satisfying for it to be a complete experience that I would rate highly.

The question is quite stupid. A series where you "loved everything", might get anything from a 7 to a 10. (a series with an 8, it still exceptionally good)
"Yeah, I'd have rated the series an 8, but due to the terrible ending, I'd give it a 9"
...

A terrible ending can easily make a series drop a point ...or several.
It all depends on how good the series is, how bad the ending is, and how much the ending affects/ruins what came before.
It depends.
There is a specific anime, where I clearly remember that I would have given it an 8, if not for the mediocre/disappointing ending, which dropped it down to a 7 for me.

9? It really depends how long it is.

Anything from 7 to 9 depending on how bad the ending was, but I wouldn't go lower. To me, the journey is just as important as the destination, so I'm not gonna forget about what came before all because of a bad ending. Finding good endings is considerably more difficult than finding good stories, honestly.

This is basically the very definition of Gantz, and I gave Gantz a 7/10, so... that's my answer 😛

Hard to say... if it’s e.g. a slice of life series with an abrupt ending, I would still rate it well. If it’s e.g. a mystery manga and the ending ties up the plot with some ludicrous explanation, I would rate it much lower.

What about if the series was terrible the entire way through, and the ending sticks the landing to the point you want to go murder someone?

The ending to Gantz isn´t more or less dumb than anything else in it. It´s only somewhat sudden. Just remember the vampires/his brother, the dumb as bricks explanation of the "game", the blackface shooting, Nishi´s bizarre non-arc(s), the mountains of outright misogyny, etc. Gantz is what a mental insufficient teenager would think of as cool or smart. That´s half the appeal.

I just watched the Watchmen finale. Not necessarily a "bad" ending but reducing it all to a cabal of cartoon racists and a crazy scientist being brought down by her own hubris is lame. The returning characters were all off canon too but you have to accept that to even get this far, I guess. And get out of here with your ambiguous final shot bullshit Lindelof. There is nothing exciting about withholding a foregone conclusion! Just finish with an exciting money shot but Damon Lindelof is psychologically incapable of writing 3rd acts. A good show of course but I reduced my Imdb rating from a 9 to an 8. Snydra´s slavishly accurate director´s cut did it better and even he stumbled through the finish line. Let´s see about Doomsday Clock, as even that one is running out of steam at the end. Alan Moore made it all look so easy.

And Game of Thrones. That show broke its legs during S5 and S6 (some of this can be blamed on the books those stumble/rush through to adapt) to then shit the bed with S7. Anyone surprised by that abortion of a final stretch wasn´t paying attention.

I had to do math to figure out my Imdb rating, just as I once did for Dexter and House of Cards. Here is how my GoT Imdb rating came about: (9+10+10+10+6+6+4+3)/8 = 7,25
All 3 ended up with a 7/10. Even fucking Dexter... Math is a harsh mistress.

I think this happened exactly once for me with this Hoshino manga we don't talk about here. I actually removed my rating from it.
If an ending is that bad while the rest is so good I could not recommend it to anyone, but I could also not say to not read it...
So if there isn't a possible rating I could give it, it's just not getting one.
And I almost feel sick even thinking about it so I'll stop here.

Remembering Tokyo ghoul re... Started great after the prequel, got meh and then had a ending that felt like a slap in the face.
So, about 7 to 8, really depends on when the decay set in.

Usually a series drops in quality prior to a really bad ending in my experience, so chances are high I dropped the manga before seeing an ending I actually hate.

Depends.
Most manga that have a crap ending don't have a crap ending that retroactively ruins the entire comic. there are a few cases where it does in fact 100% ruin it though, in cases where it doesn't, 6-8 mostly falling in 7 in cases where it does, I wish I had brain bleach.

If it was axed then I ignore the ending completely. For other it depends how much it dragged and changed the rest. If it's just the ending it might be an 8 or 9 but if it maked me change my opinion of other parts of the series it might be a 5 or 6