Each year, the Environmental Working Group conducts an extensive study to determine which produce are the most and least contaminated. From the data, they compile a list of which should be eaten organic and which are OK to eat conventional.
Buy these organic (in order of most to least contaminated):
peaches, apples, bell peppers, celery, nectarines, strawberries, cherries, pears, grapes (imported), spinach, lettuce, potatoes
Conventionally grown OK (in order of cleanest to least clean):
onion, avocado, sweet corn (frozen), pineapple, mango, asparagus, peas (frozen), kiwi fruit, bananas, cabbage, broccoli, papaya.
If you want the full list, you can download it from http://www.ewg.org/sites/foodnews/walletguide.php