As others have said it depends on what you like to cook, but here are a few items that are on my grocery list every week....without fail:
onions (usually white and red, yellow if they're available and look good)
variety of fresh veggies such as green beans, okra, carrots, squash, carrots, bell peppers, etc
When I moved to Kuwait for my new job back in October last year, I had spent the previous 5.5 years in Iraq where I lived on a military base and had no kitchen in my trailer. So my first 2-3 trips to the grocery store were enormous....as I was stocking up my kitchen. My boss gave me a lift to the store those first few times as I didn't have a car yet. She doesn't cook at ALL and was shocked to see what all I was buying.
Yet these were the kinds of things you might not buy every week or need all the time, but when you need them you need them. In my opinion, every kitchen should have:
sugar (all varieties)
salt - sea salt and course ground are my favorites
the 3 peppers - black, white (my fave to cook with) and red
red pepper flakes
onion powder & garlic powder (not the salt versions)
There's more, but that's all off the top of my head.