Depends what you want. Santorini is spectacular, has some amazing hotels that would be good for a memorable stay, good restaurants (a bit of a rarity in most places), but as has been mentioned, it can be very touristy in the main town. The famous beaches are packed but there are plenty of quiet ones.
Sifnos is a place where Greek families go, and we had great holidays there when Miss J was a nipper.
Another place to think about is Andros – also fairly quiet, has an improbably fantastic art gallery, and easily accessible (from Rafina, not Piraeus).