Here's a list of the Top Twenty Worst that I have memorized and only buy organic:
1. Celery (no, it's not as pretty as the chemically treated one)
2. Strawberries
3. Peaches
4. Apples
5. Blueberries
6. Nectarines
7. Bell Peppers
8. Spinach
9. Kale/Collard Greens
10. Potatoes
11. Grapes
12. Lettuce
13. Blueberries
14. Carrots
15. Green Beans
16. Pears
17. Plums
18. Summer Squash
19. Cucumbers
20. Cantaloupe
There's more (I think the full list goes to 50 if you google pesticides and fruit/vegetables) and one that didn't make the list is beets, which I only buy organic - not because of pesticides but because of GMO's (genetically modified organisms) which I try to avoid like the plague...