Skip Athens and Thessaloniki, everywhere else is beautiful in Greece. Generally as a Greek I prefer islands to mainland. Better climate, not so hot in summer and milder during winter. my top 10 islands: CRETE, CORFU, AMORGOS, NAXOS, MILOS, RHODES, SAMOS, PAXOS, LEFKADA, FOLEGANDROS. But I am sure there are lot of beautiful islands that I haven't visited yet: kefalonia, Zakinthos, kalymnos, karpathos, kythira...etc