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.