All your basic staples: salt, flour, oil, sugar, pasta, pasta, milk, eggs etc. There’s literally nothing to do better or worse, so for god’s sake don’t pay for the label. Fancy olive oil is nicer, and fancy butter for actually putting on bread is nice too - but for cooking, cheap the hell out.
Get your spices from an Indian / Asian / etc grocer - you can get a huge bag for the price of a tiny supermarket jar, and because they have so much turnover, they’ll be plenty fresh.
Store-brand laundry detergent and dishwasher tablets work just fine for me (and dear god you can save a lot on those).