I tend to mix various supermarkets.
Aldi for some things ... they're particularly good for organic vegetables, bin bags (trash bags), cleaning materials, and some speciality items.
Tesco or Dunnes (main stream supermarkets) for branded items e.g. shampoo, conditioners, laundry products, various food items, sauces, cooking sauces etc etc.
Local bakery for bread - it's a million times better than anything you can get elsewhere.
Marks and Spencers (very high end supermarket) for some extra nice bits, and pre-prepared food.
Cork, my home town, also has a huge indoor market which has an amazing selection of pretty much everything - it's quite gourmet but, it's also realatively cost effective so I go there for : meat, fish, fresh pasta, some vegetables, charcuterie, confectionary, wine, etc etc.
That's pretty much how I shop.
Going to a single middle-of-the-road supermarket is pretty dull and it's kinda fun if you have the time to shop properly.
I would tend to shop for fresh items e.g. meat/fish and some vegetables and herbs on the day of consumption, or one day ahead a most.