Like Taun, I have visited Longleat, Knowsley and West Midlands
Longleat:
It's setting is magnificent (in the grounds of a manor) and it does contain some great species (mostly standard safari types). There are some good enclosures too (flamingo and vulture aviaries as examples). The one thing that I would pull them up on is the very odd mixing (Rhino & bactrian camels, sealions & hippos).
Knowsley:
It was OK, a good amount of space and some very unusual safari park species (congo buffalo, bongo etc) Cape Buffalo are also a highlight. Another plus point is that the elephants have a very large area to roam around. I don't really like the fact that the giraffes have a seperate, small, paddock when the other two parks i mentioned keep them in the giant drive-thoughs (with no apparent problems).
West Midlands:
In my opinion, it is the best of the three. Sensible mixed reserves:
African - White Rhino, Giraffe, Zebra, Wildebeest, Eland etc
Asian - Water Buffalo, Philippine Deer, Banteng, Blackbuck
They also have a good range of the carnivores (lion, tiger, african wild dog, timber wolves, cheetah, leopards). A large hippo group, cape buffalo and elephants are also part of the impressive collection. WMSP also has a good nocturnal walk-through and very interesting insect and fish buildings. The only negatives are the amount of traffic and the white lions and tigers (which I'm not in favour of). Even the sealion show is well done!