Beans (the dried kind in the bags, so much better than canned). Red ones for veggie chili, navy beans for vegetarian baked beans (they are delicious home made, and nothing like the gross syrupy ones in cans), lentils.
Tofu to drain and season and bake for Buddha bowls. There is an awesome NYT recipe for this kind of thing:
https://cooking.nytimes.com/recipes/1024956-sweet-chile-grain-bowl-with-tofu?unlocked_article_code=1.d00.u_Bp.lInem-SRV5Ph&smid=share-url
Eggs. For omelets, scrambles, deviled eggs, and egg salad. Also for baking. I rely pretty heavily on eggs.
Canned tomatoes. For making pasta sauce. For adding to beans and veggies for chili. I also like just eating canned stewed tomatoes.
Starches: Rice/potatoes/pasta
Cheese.
Fresh veggies I always have on hand: onions, garlic, green pepper, tomatoes, celery, carrots, salad greens. Additional veggies for whatever I want to make, or is in season.
Fresh fruit: I always have apples/oranges or tangerines/bananas in a fruit bowl. And often have berries and/or melon in the fridge.
Frozen veggies. Frozen fruit.
Nuts and seeds. Pecans and/or walnuts for snacking. Chia seeds for chia pudding made with chia, almond milk, and frozen blueberries for breakfasts.
Cream of Wheat or hot rice cereal. I like this with honey or brown sugar, but I also like it savory with salt, pepper, butter and cheese topped with scallions.