Aside from being in the grocery store at all, usually the people who have absolutely no awareness that they aren't the only person in the store. The ones who stand with a cart diagonally across an aisle blocking everything, or the people who stand side by side chatting and blocking the aisle.
When we were trying to keep six feet of separation during COVID times there were the people who acted as if they inherently owned every inch of space within six feet of them and would take their sweet time standing at a shelf pondering what they wanted. It didn't take very long for me to just walk up and take what I wanted off the shelf. If you don't like me being close to you, get on with it or step aside for a minute.