This is one of those questions for which everyone will have a different answer. So much depends on your experience and comfort level and the time of year that you plan to visit. The good news is that the Greek Islands offer something for everyone, from the Ionian in the west, where it is very crowded in season but with mostly benign conditions, to the Dodecanese in the east, where it can be very windy in the summer. And then there's everything in between. There is a article that gives a brief background to the various areas in the link below:
But wherever you decide, you won't be sorry you visited. Enjoy!