Top 5 best zoos in UK I have visited:
Chester
Colchester
Edinburgh - only here as Bristol closed but based on a 15 year old memory.
Paignton
Cotswolds WP
Honorary mentions: Whipsnade and London. Jersey would be on the list, probably 3rd/4th depending on how we are classifying the Channel Islands.
It may be that London and/or Whipsnade should be on the list though I am not a fan of what the ZSL collections have become in recent years. London in 2005ish? would be 3rd, back when the Clore Pavilion was still reasonably diverse.
Top 5 favourite zoos
Chester
Hamerton
Cotswolds WP
Paignton
Newquay
Honorary mentions: Beale, Banham, Marwell.
Disclaimer: I have not (yet) been to either Aspinall collection, Longleat, Exmoor, Blackpool, Dudley or YWP.
Exmoor is high on my list and based on how it comes across in images and others reviews of it I imagine it will feature in both lists!