I agree it is a beast, and it has loads of hairpins and 40% of it is unpaved. Oh and its 9.2% btw, even worse
I think generally regarded Finnestre is knowhere yet though.
It would be like this- "general opinion".
1 Mortirolo
2 Angliru
3 Ventoux
4 Zoncolan
Forum opinion
1 Zoncolan
2 Angliru
3 Mortirolo
4 Kronplatz
5 Finnestre
6 Stelvio
My opinion

1 Zoncolan
2 Angliru
3 Finnestre
4 Crostis.
5 Mortirolo
As far as "generally regarded" goes, I do not think that correlates too strongly with the actual difficulty order.
Generally I think Mortirolo is regarded as the toughest climb. Its been around a lot longer than Zoncolan, quite a bit longer than Angliru and has always been regarded as super tough. Mortirolo has been used 10 times and had a decade to build up its number 1 status until Angliru was introduced.
Zoncolan Ovaro has been used only 3 times and the first time in 2007. Even though its had 1 hell of an impact thats still a very short time to suddenly get regarded as the hardest. Moreover Zoncolan is just another monster the Giro has in its arsenal, and just another huge introduction by a race which seems to come up with tougher climbs every few years.
Angliru has been used twice more than Zoncolan OVaro, been around for over a decade now, and is seen as that Vuelta climb. By far the hardest the 3rd gt has to offer and always produces carnage.
Finnestre has only been used twice, and while the first one will possibly be 1 of the best remembered stages in cycling history, it needs a few more gc deciding climbs of it to go down in mountain folklore next to Mortirolo, Angliru Zoncolan.
On the forum though, during the Giro last year 1 or 2 posters said they thought Finnestre was harder than Zoncolan and than Crostis, and 1 person said he thought Finnestre alone made the otherwise tame stage 20, the queen stage. 1 calculated using his own equation that Finnestre was the hardest mountain in the world followed by Crostis Zoncolan and Angliru.
But it should be mentioned that all the other climbs held in such high regard are short and steep regularly go above 10% and none of them is longer than 12 or 13km. When it comes to long climbs Finnestre and Stelvio are easily the hardest.