I stayed at all three.

My favorite by far of those three is the PH Milan. This might be skewed because I got the best upgrade at that stay vs the other two.

I liked the Paris, thought the Zurich one was nice but not spectacular. IMO the Zurich PH is only a category 7 only because Zurich is really expensive in general.

If you are looking at bathrooms alone, the Paris has the best of the three (love the heated floors). Milan has a great bathroom too. Zurich really was just a standard nice hotel bathroom with nothing that wowed me.

It also depends on your status at Hyatt? Do you have Diamond or Platinum?

Being a Diamond for instance will greatly change your experience at those hotels.

I never went to PH Istanbul, but the PH Vienna is spectacular also. I think Milan will be the only one up to that standard.

I also stayed at Andaz Amsterdam, and it is a nice hotel but definitely not a Park Hyatt. For a frame of reference though, I thought it was on the same level as Park Hyatt Zurich to me in terms of experience.