I actually prefer smaller zoos to large zoos, I think they have more character and are far more interesting as they can't fit everything in and have to be more selective making them different from each other.
I really like Bristol zoo (whihc I would rank above London zoo) with a good choice of species and excellent exhibits (such as monkey jungle and Penguin and seal coast). Newquay zoo has a really interesting collection including Paca, Black wildebeast, pied tamarins, fossa, owstons palm civet, Carpathian lynx, narrow striped mongoose (not yet on show but planned), Merriams kangaroo rat and visayan warty pigs among others (but these are the most unusual/unique)