The "Dirty Dozen" is a list of twelve fruits and vegetables that should be purchased and / or grown organically because they typically are sprayed with more pesticides, insecticides, etc.; are grown in heavily treated soil; don't have thick peels to protect them; and / or retain many of the chemicals used, more so than other produce.
I would also add all baby food, corn and soybeans to this list. Baby food, if possible, should always be bought organically or made with organic ingredients. An infant's immune system is not as developed as an adults and can't handle the onslaught of chemicals. They also will retain more of the chemicals which can manifest in a lot of different, destructive ways such as reduced immunity. Corn and soybeans are almost always genetically modified but when bought organically (especially if an heirloom variety) genetic modification can be avoided. For a great look into this watch Food, Inc. (http://www.foodincmovie.com/)
Foods that are relatively clean of pesticides (though they may be heavily fertilized depending on the farmer) are:
* Sweet potato is controversial as it is a root vegetable and may pick up many more fertilizers from the soil than other foods.
It is always best to buy these foods as local as possible to reduce the number of pesticides sprayed, especially on those that travel great distances and thus need to be protected from bacteria and bug infestation. Buying local also reduces the carbon footprint (what it takes to get food from seed to plate) in the form of gas, packaging, manpower, storage, etc.