I always make sure I have the following at all times:
Tinned tomatoes: whole, crushed (passata), chopped, paste
Tinned beans: chickpea, red kidney, white kidney, pinto
Dry lentils
Couscous regular and Israeli
Dry pasta: spaghetti, farfalle, penne, ziti, cavatappi and orzo
Dry noodles: ramen, udon, soba
Rice: long grain white and brown, basmati white and brown, jasmine, Arborio
Panko
Oats
Baking essentials: bread flour, white and brown, self-rising, plain, cornmeal, nixtamalized corn flour (masa)
Yeast, baking powder, baking soda
Sugar
Salts: finishing, Kosher, Himalayan, smoked, table
Plenty of spices
Dry chiles
Oils and vinegars
Dry fruits
Nuts and seeds
Popping corn for snacks
Dark chocolate powder for baking and a package of mini dark chocolate chips for baking and snacking 