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:
chicken
fresh fish
onions (usually white and red, yellow if they're available and look good)
fresh garlic
lemons
butter
eggs
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:
flour
sugar (all varieties)
corn meal
yeast
olive oil
vegetable oil
peanut oil
sesame oil
chili oil
salt - sea salt and course ground are my favorites
the 3 peppers - black, white (my fave to cook with) and red
red pepper flakes
oregano
basil
thyme
rosemary
curry
ginger
nutmeg
chili powder
onion powder & garlic powder (not the salt versions)
cinnamon
vanilla extract
almond extract
There's more, but that's all off the top of my head.