I tend to find somewhere I like and then keep going back there, so my list's not as long as I'd like it to be, but here goes:-
Paxos (12 times so far) - 10/10 Just the most beautiful place I can imagine.
Zakynthos ((6 times in the 80's/90's) - Started out as a 10, but during my last couple of visits it got marked down and I'd probably now put it at about a 5, purely because of the way over-development has changed it.
Kefalonia (twice) - 8/10 A big, beautiful island with lots to see and do.
Corfu (twice) - 8/10 I'd give Corfu Town and some of the resorts a 10, but marked it down overall because of places like Kavos and some other "English pubs and fish & chips" type resorts.
Samos (once) - 8/10 Another beautiful island with plenty of different places to explore.
Crete (once) - 7/10 Only went there for a week out of season, so didn't really see it at its best. Would like to go back and see more though.
Tolon - 7/10 A bit too cosmopolitan and "ungreek" for my tastes, but good for nightlife and plenty of places of interest nearby.
Ag Theodora (near Corinth) - 7/10 My first taste of Greece back in the 70's. Loved it then, but it's probably changed a lot since!