The nonprofit organization Environmental Working Group regularly examines pesticide levels of various commercially grown produce. According to their most recent list, the following fruits and vegetables are the ones most likely to be contaminated. So it’s definitely in your best interest to try to switch to organic versions of these items:

peaches
apples
bell peppers
celery
nectarines
strawberries
cherries
lettuce
grapes
spinach

Now for the good news: there are some fruits and vegetables that didn’t rank as high on the pesticide scale. These items are ones that you can afford (speaking from a health perspective) to buy the “regular” versions of:

onions
corn (frozen)
avocado
pineapple
mango
sweet peas (frozen)
asparagus
kiwi
bananas
cabbage
broccoli
eggplant

More info: Environmental Working Group