If you're looking for a "wow" factor, I'd look to the Cavalieri (Waldorf) in Rome. It is outside the city a little bit - but that's nothing more than a 12Euro cab ride up the hill, and you feel like you're up over the city looking down. The service and hotel are amazing.

I've stayed at 5 or 6 of the nicest properties in Rome and the Cavalieri is my favorite of them all.

But since you transferred to SPG, between the Saint Regis and the Excelsior, I'd choose the Excelsior only because the service is a little less stuffy but the hotel is still beautiful.