I think the deep concrete pit which housed Whipsnade's Kodiak bears (and which once held tigers) was probably the worst exhibit at Whipsnade.
I would agree completely: except that the polar bear enclosure near by was a little worse in my opinion, as it seemed even smaller and was surrounded with black bars with curved tops that ended in downward pointing spikes.
Continuing with this digression from London Zoo:-
I agree that Whipsnade's polar bear exhibit was very small, and uninspiring, but I think the Kodiak bear exhibit was worse both for the bears and the visitors.
The pit was deep so the Kodiak bears could only be viewed from above and bears couldn't see outside the confines of their pit.