That is the question.

The Environmental Working Group (EWG) used the results of nearly 43,000 tests for pesticides on produce collected by the United States Department of Agriculture (USDA) and the U.S. Food and Drug Administration (FDA) between 2000 and 2004, to produce a ranking of commonly eaten fruits & vegetables.^[17]  Wikidpedia, 2012. 

The EWG lists the “Dirty Dozen” and a “Clean 15” based on pesticide tests from the U.S. Department of Agriculture and the FDA.  Remember that pesticides are designed to withstand rain, and often don’t wash off with plain water.  Using a diluted solution of vinegar can remove most residues, but some get into the plant and can’t be washed out.  Strawberries are an example of this.

This EatingWell article illustrates the best produce to buy organic, picks that are on the fence, and produce that is considered conventional and safe.