The Green Guide publishes the Top 10 Most Important Foods to Buy Organic, based on information from the Environmental Working Group. Here's a summary:
1. Baby Foods
8. Green Beans
9. Apples, apple juice
And the runners-up are: nectarines, grapes & raisins, and kiwi fruit.
Elsewhere I have read that buying organic meat and dairy, rather than fruits and vegetables, gets you the most "bang" for your grocery "buck".