Fun to see people posting again here! In 2015, I said VLV, which is still my personal favorite. I think you can make a good argument for almost every Coldplay album except MOTS, which I think is just objectively mediocre.
In 2021, I think fans should start considering Everyday Life. Not everyone loved it initially, and even I had issues with the pacing and how some themes seems only lightly touched on in a somewhat superficial way (Trouble in Town, Guns, Eko). But now, I start to understand Chris' vision for the album a bit more and because all the songs are so damn good and the collaborations are so cool, I would maybe add EL to the candidate list along with VLV, AROBTTH, and Parachutes.
I think that all boils down to what we intend with the word "masterpiece", because there are several albums in their production which could share that title. I find it tricky to think about this in absolute terms. Plus the album is the final outcome of an era, and this makes things more complex.
To me, Viva was their masterpiece. However there are other albums which are gems in their own ways and come close second.
I don't like every album of theirs, but I want to think that for every album they have tried their best. I know that they will never give a definitive answer, but it would be cool to ask what the band thinks about this. After all, it's their work, so they could give quite a different vision from the generally shared one! (but I suspect their answer wouldn't be x and y, as far as we know😅). Or maybe they would say their masterpiece is yet to come, who knows