I would actually place Co-Op above Tesco at the moment. Co-Op is a little bit dearer at times for not being that high quality, but I think their own brand is better than Tesco's. I've been very disappointed with Tesco recently.
Co-Ops are also cleaner, neater and easier to find stuff in than Tesco, which is often a stuffhole in comparison (at least around me).
My favourite is Sainsburys. It's both affordable and of decent quality, plus they have multiple ranges of varying quality for when you want to treat yourself. Their 'Taste The Difference' is really high quality.
ASDA is actually a bit surprising in being quite a decent quality and at a really good value. It's not surprising that it has kept Tesco and Sainsburys on their toes, although it does have the advantage of having a multi-billion dollar international mega-corporation behind it (before being bought by WalMart it was moments away from bankruptcy).
The big downside of ASDA is that it is very very American and they expect staff to interact with customers in a very cheesy and inauthentic way. It's not very British.
Lidl's are of an unexpectedly decent quality, at least in that they can be very affordable on branded goods. But they do sell a lot of stuff, are very untidy, and usually short staffed. Their staff are paid really well but they do work really hard for it.
I have little experience with the rest. I've never been in a Waitrose but know it's more expensive, but don't know if it's justified.
Marks & Spencers is overpriced for no substantial increase in quality. It also feels odd to buy groceries in a department store.
Morrisons is much the same in terms of pricing, and I'm still shocked that some friends of mine who were at university shopped there when there was cheaper available.
My parents went to an Aldi recently and said it was such a mess that they couldn't bother being in there.
