The Environmental Working Group, an organization that studies pesticide contamination, ranks onions and avocados as the most pesticide-free vegetable and fruit, respectively—even when grown conventionally.
In fact, as a general rule, anything you have to peel before you eat (such as bananas or garlic, for example) is relatively low in pesticides. If you want to eat organic, splurge on produce with permeable or edible skin, such as peaches, lettuce, and apples.