If you want to grow a lot of food, you need rain and hot temperatures ,ideally all year round. If it was possible, I would choose an island with an equatorial climate. The climate of Indonesia would be very good to support large populations within a small area. It's possible to have 2 or 3 harvests per year with some crops like rice once you get settled.
As Frostfyre mentioned, having some volcanoes around would have enhanced soil fertility over time.
One problem I see with this is adaptability. It's an isolated island, probably far from the other landmasses and people possibly lived in another climate. They might find it too hot/ too humid. They might not be immune to the local diseases. They might not know what plants are edible and even less how to cultivate them.