Best Tiger Enclosure - Whipsnades Tiger Enclosure + Howletts has been considered by quite alot of people do be very good too.
I am going to continue championing the cause of the IOW's Tiger Sanctuary. Although only stages 1, 2 and 3 have so far been completed, they provide excellent facillities for the animals and each enclosure is based on an actual national park in India (so far Kanha, Ranthambhore and Penna). Each have different features inspired by the parks which they are based on so that, when all five stages are complete, the 20+ tigers will be rotated and have different environments and enrichment oppurtunities each day.
Also, the zoo contributes to the conservation of tigers in India, to the breeding programme by taking in cross-bred tigers, and is the only place where I have seen the white tiger story successfully explained to the public.
I regard the Tiger Sanctuary concept and its enclosures as more innovative and better for the public and animals then both Whipsnade's enclosure and (sorry Riziki) the Howletts enclosures.
Just to finish, here are the plans for the astonishing stage five, based on Bandhavgarh national park. I am quite sure nothing like this has ever been attempted before...
Future developments at the Isle of Wight Zoo