For a well located 3 star property I can recommend the Philippos (http://www.philipposhotel.gr/)
It is walking distance to the Metro,Acropolis Museum and the Plaka.It is similar to a Novotel and is in a quiet local neighbourhood.
The same company has a 4 star property around the corner called the Herodion which has a great rooftop bar looking up to the Parthenon. I did not stay there but it looked perfectly OK.
I do not know the Melia.
If you are taking an early ferry to Mykonos then you might want to consider the Hotel Phidias in Piraeus.It is similar to the Philippos and has a van that will deliver you to the gangplank of your ferry whatever the hour. Pireaus is more blue collar than central Athens but still has markets and restaurants that are worth checking out.