Going to out myself as an absolute snob.
I travel around London for work, so have seen pretty much all supermarkets.
Here are the results of my extensive research 
Lidl & Aldi
Grim, grim places. Absolutely hate going in there. A lot of fellow Germans go in there, for the german stuff...but I just go the Polish shop. Cleaner, brighter. Staff gets treated apalingly, check out line absolutely annoying. I only went into my local one once but emerged with a filled pepper mill for 89p. So not all bad.
ASDA
Hate the lighting, hate the fact that most of them have mostly self-check out lanes, and last time I was in there you couldn't even turn the volume off. Staff also don't get treated very well. I can't really comment on their own brand stuff, though but it looks like it was made on the cheap.
Morrisons
Used to live a really nicely-laid out one. Pretty cheap for own brand and brand stuff. But their own brand stuff is absolutely vile, as are fruit and veggies.
TESCO
Usually I'm not a fan, the shops seem messy, the staff clueless. Their own-brand range can be pretty hit-and-miss. There is one shining exception, a 24 hour tesco in North London. Their fruit and veg, meet and Deli produce is really good and they have amazing herbs. The sometimes have my favourite Haribo's (not normally available in Britain). Loosing points for having a pretty crappy record when it comes to employment terms.
Sainsburys
Very good own brand ranges. Even basics are pretty good, TTD products are in many cases better than branded stuff. Very good for organic produce. Very good deli cheese and cold meets sortiment (though staff at two different stores were confused at me asking for something measured out in grams).
Waitrose
My favourite supermarket in the world. All their own brand stuff is amazing. Winner for quality and ethics. Their fresh bread is great, and they sell rye bread (yay!). I love Duchy originals, especially their milk is great (make a point of only getting DO milk). Amazing quality. Best choice for free range meet (amazingly not only chicken). It is not that much more expensive thank other supermarkets (with their loyalty card it often ends up cheaper). Staff are treated well (as far as i know). If I would have to find a fault, the shoppers tend to be on the entitled side, can be a bit slow, and tantruming children tend to not be stopped (freerange children, after all)
Oh, yes and their loyalty card. Free coffee, free newspaper, instant savings on your shopping.
Sadly Waitrose have decided not to pay me for that little bit of free advertising.