The Grande Bretagne is a great example of a classic luxury hotel. I am a Starwood loyalist and it's my favourite Starwood hotel in Europe. It's not comparable to the top luxury chains, but particularly if you book a suite and are served by the team of Butlers you can't ask for a better base in Athens.
My favourite hint: For dinner, skip the busy but uninspiring Grand Bretagne restaurant and head next-door to the King George's Tudor Hall, which you can add to your hotel bill. Still my favourite Athens restaurant, despite having been to others which are better regarded.