It really depends what you want, and when you’re going. Early May is quite different from late June.
If you want the classic Greek island with white houses, blue roofed churches and bougainvillea then pick the Cyclades islands (generally best to fly to Santorini/Mykonos and get a ferry to your chosen island from there).
The Ionian islands are more Italian feeling, tiled roofs, greener. Many have their own airports if you prefer a simple journey.
If you want lots of walking and countryside then Crete is probably best, though Amorgos and a few others are also known for this. Crete is lovely but for me doesn’t have that classic Greek island feel as it’s so big.
If you want sandy beaches that narrows it down a fair bit - Skiathos, Kos, Kefalonia, Zakynthos, Paros all have them.
My own favourite is Alonissos - a classic Greek town (Chora), beautiful water (but stony beaches) and not too developed. Paxos is also lovely.
On the bigger islands be careful which area you pick as some can have party central and quiet and authentic, all on the same island but feels a million miles apart.