Firstly, the exhibit shown for Exmoor currently holds Yellow Throated Marten, not Tayra. The Tayra are held next to the Sand Cats, and opposite the Binturong (I'll try and find a photo somewhere after posting this).
Keeping everything in mind, here's how I'd rank the exhibits - Exmoor and Hamerton probably tie for the top spot.
1/2. Exmoor - a great all rounder. Good viewing, lots of foliage and climbing opportunities, but not the largest.
1/2. Hamerton - likely the largest exhibit, with lots of climbing opportunities, but a little bare in terms of plant life. A very commendable breeding record too.
3. All Things Wild - A decent all rounder again. With multiple exhibits, and a large on show indoor section. Since the photos shown here were taken, more climbing structures and plants have been added. However, it doesn't provide much height, like Exmoor or Hamerton do.
4. Axe Valley - relatively new and built "in house", but good quality. Lots of branches for climbing, and larger than the photos make it out to be (but it's still not large). The clear pannels also make viewing better than some other exhibits. However, it's lacking any greenery, is fairly low, and only their indoor exhibit offers restbite from the public.
5. Hemsley Conservation Centre - Whilst I've not seen this exhibit in person like I have the others, from photos I've seen and from speaking to those that have visited, the build quality is very poor. The space doesn't seem to have been utilised as well as the other exhibits, but can be commended for featuring some plant life.