The aisle blockers are my biggest peeve - 2 people going in opposite directions just have to stop and chat and get annoyed if you politely ask to get by. Maybe they would relate more to being rude to them.
The out of control screaming kids being screamed at by their out of control screaming parents. I don't know which is worse!!!
The "ankle busters" - following so close behind you that when you stop - you get rammed with their cart. OUCH!
And of course don't forget the baggers with not an ounce of common sense who pack the cleaning products in with the food products, or the shampoo in with the eggs - only to discover when you get home that the top of the shampoo bottle popped open. Get the picture on that one???