I have stayed in all 3. I used to prefer King George because it is smaller and with great views from its dining terrace of the Acropolis. However, it was privately owned, but the hotel was taken over by the owners of Grande Bretagne, so I am not sure if service will be as good as it was.

Grande Bretagne is a typical Starwood property - overly large for my taste and with nothing special on the food side of things. Rooms/suites are OK, but you must take Club floor accommodation, which is better than most - a little butler service, too, though nothing too special.

The Hilton is way past its sell-by date, but it does have amazing views from its bar and terrace. Not worth staying there though for this.