No particular order, and probably no surprises here -
Exmoor - probably the best selection of unusual small and furry things in the country, and small furry things are my Kryptonite. Ratel, Fanaloka, Tayra, Yellow-throated Marten,so many cats... Just great!
Birmingham Wildlife Conservation Park - the zoo I've visited most, my childhood collection, and a place where the keepers know me by name. It's only small but perfectly formed, and some very choice species, and the reptile collection and nocturnal buildings are definitely the highlights but there's still a lot of other stuff to see on a 6.5 acre site. Probably only on here for nostalgic purposes but it's my list so it stays.
Chester - simply the best this country has to offer, a well-rounded collection with most of the big-hitters and loads of unusual stuff as well. Not perfect, and those of us with a cynical eye are watching the new ventures with caution, but at the moment it's still a great day out.
Cotswold Wildlife Park - charming, elegant, and peaceful. Probably my favourite place to spend time, especially on a warm autumn afternoon. The Walled Garden is a zoo within a zoo, Madagascar is propably the best walk-through in the country (sifaka definitely swinging it), the rhino's on the lawn is the most iconic zoo view I can think of, and even the restaurant has Gundis galore! Add in a well rounded collection and it's close to a perfect half day zoo.
Dudley - another one where nostalgia is definitely doing a lot of the heavy lifting, but again, my list, my rules!

To be fair the zoo is a place that's far removed from the place I went to as a child, and still strives to improve. It uses a lot of its older exhibits well and embraces its history where it can. Also holds many memories for me so it was always going to squeeze in.
Honourable mentions go to Hamerton, Paignton, Shaldon, Whipsnade, and London.