Let's pretend that they are just great for conservation and there are no negatives.
Went to Marlow Zoo yesterday (lovely day for it). It was ok but not a patch on either Edinburgh or London. They weren't ready for the weather either and didn't have enough concession stands open so I didn't get an ice cream! Lack of monkeys, no lions or elephants, no aquarium etc. Penguins were in good form, strong meerkat enclosure, large amount of lemurs, lovely otters.
Which zoos are best? Let's keep this to the UK and not world zoo grandstanding (although I have been to San Diego - twice!).