I use Tesco regularly, Sainsburys about once a month, Amazon Pantry & Waitrose a few times a year.
Sainsburys is generally not the cheapest on everyday items & if they give you a substitution you have to pay the full price for it, then get a voucher off your next shop for the difference.
I hate this complicated way of doing things enough that I only use them rarely for a couple of specific items the other stores don't stock.
They also insist on handing you coupons with your online delivery that can only be used in store in the next couple of days.
I don't even know what year it was last time I was inside a Sainsburys.
However, they also give you more coupons than the other stores seem to.
I currently have £6 off a £40 shop coupons online for the next 3 weeks.
Waitrose take the most care of my shopping & have given me some really great substitutions on the odd occasion that they need to sub but it's a £60 minimum spend & most products you can get elsewhere are more expensive.
They stock things I can't get anywhere else locally like Pasta Flour.
Their products tend to be shorter dated on things like fresh milk where I live.
I like Waitrose but they annoy me by giving me random things like a free newspaper I don't want & a free full colour magazine I also don't want in the same delivery.
They both go straight in the recycling bin after the delivery guy leaves.
Tesco is the one I use the most. £40 minimum order.
They tend to have better prices & give me better substitutes than Sainsburys.
Longest shelf life on things like Milk out of the three supermarkets (that may not be true elsewhere in the country but it is round here. There can be up to a week less on the use by date between the different stores.)
I use some of my Clubcard vouchers on Triple boost (or whatever they call it now) to pay for my delivery saver so I regard my deliveries as free.
I sometimes find free things when doing my Tesco shop, e.g. last week I got Innocent Almond Milk free, a few weeks before it was something else. They generally tend to be yoghurt or cereal things.
I did try Asda a few times, they were a complete nightmare. £40 min shop too.
Site issues meant I couldn't add milk to my order for 3 days as it wouldn't let me log in & they wouldn't let me phone their customer service up to add it or to cancel the order.
Food that did turn up very short dated, worst quality fruit & veg I've ever seen that goes off faster than any other store.
When there is a problem, they don't care or do anything about it.
That may be a problem with my local store, I don't know.
If you have Amazon Prime you can use Pantry, or depending where you live they have other store options that do fresh items (not in my area) It's worth checking for store cupboard or household items as sometimes they have good prices.
It's also the only way I can get Morrisons items as my local Morrisons doesn't deliver here.
If it's your first shop, there is usually a code you can get to get a discount off with most of the online shops but it is not always worth it if you usually do small shops.