I've stayed at Istanbul, Milan, Vienna and Paris. In terms of hard product Vienna and Istanbul are light years ahead of Paris and Milan. I'm pretty sure that both Paris and Milan used Ed Tuttle as their architect... And that they are of similar vintages (turn of the century?) neither of those hotels offer then same experience of Istanbul and Vienna. But; they do offer Paris and Milan. Both have amazing locations and solid service. For me it would come down to which city I wanted to spend more time in. For me that'll always be Paris.

Fdw
Note I said nothing about Zurich. I've been to Zurich probably half a dozen times. It's an incredibly clean but kind of sterile city. Very very exquisitely expensive. Ild steer clear unless you had to be there.