Celery

"When researchers at the Environmental Working Group (EWG) analyzed
89,000 produce-pesticide tests to determine the most contaminated fruits
and vegetables, celery topped the chart. "In terms of the sheer number of
chemicals, it was the worst," Lunder says. Celery stalks are very porous,
so they retain the pesticides they're sprayed with- up to 13 of them,
according to the EWG analysis. Lunder also advises buying organic bell
peppers, spinach, and potatoes because they scored high for pesticides,
as well."

-Health Magazine April 2011