I work in a whole foods organic market and basically anything that is higher in fat concentration or comes from an animal should definately be sourced organically. As far as veggies and fruit go the top ones to source organically are berries, apples, citris and bananas, root vegetables such as beets and carrots. Grains are really important too because many of our grains come from underdeveloped countries where sanitation and standards aren't the same. But if the grain is organic then the farm does practice proper sanitation and soil standards. Many veg like Kale and brocolli cauliflower ect. will be sprayed because of the higher risk for worm infestation. If it is at all possible to shop at a smaller grocery, one that the owner actually meets the farmer who grows the food, you may be able to ask what the farmer uses for fertilizer and where the farm is located. What I find sometimes is that some of the smaller farms cannot afford to certify themselves as organic but actually practice organic farming standards.
So to sum up, definately butter, cheese, youghurt, seeds nuts and most fruits. Buy in season and from your bioregion and things become quite inexpensive that way. The further the food has to travel to get to you the more expensive it will be.