OP, there are a vast number of places to choose from, as you've discovered. You really need to decide what is most important to you for your holiday and then work around that.
For example: how long a transfer do you want from the airport and are you prepared to include a short boat trip to get to your destination? Which time of year are you travelling? May and September are gorgeous and October in Crete is also still very warm. Do you want stunning beaches, history or both? Do you want little windy streets with plenty of shops or somewhere away from it all? Once you decide those factors, it will be easier to narrow down your choices.
So, bearing that in mind: Parga on the mainland is fabulous (don’t assume that it has to be an island for your holiday), lovely beaches, shops and plenty of places to visit nearby . Paxos is also super but you need to fly to Corfu and then transfer by boat or hydrofoil. The northern Aegean is lovely, especially Skiathos (fabulous beaches, superb restaurants and nice shops, but I noticed in September it had become more expensive than before); Skopelos; Pelion on the mainland nearby is stunning but quiet; Molyvos in Lesvos looks like a film set; Samos is great. These all have little harbour villages and are very green islands.
Crete is like a country on its own and has everything.
Mykonos and Santorini are expensive and overrun with tourists (lots of day trippers from cruise ships), Paros or Naxos would be better but involve a short boat transfer. A lot of Rhodes has sadly been spoiled - Lindos is lovely: I wouldn’t go in high season, outside that it should be fine. Rhodes Old Town is a World Heritage Site but again overrun with day trippers.
If you want ancient history, go to the Peloponnese (or Crete).
I’ve been going for 30 years and think it’s the best country in Europe.