San Diego Wild Animal Park is undoubtedly the best, but it seems to me they do not have as many species in the main open field exhibit (East Africa) as they used to. And now the main tram (that replaced the monorail) does not go into the remaining Asian field exhibit or near the sheep and goat hill. The second Asian field exhibit was converted to Central African and has like two species of antelope - really disappointing.
In spite of this, it is still probably the best hoofstock collection and overall one of the best zoological parks anywhere.
If you are interested in strictly North American hoofstock, I think the tram ride at Northwest Trek would be hard to beat. It goes through several hundred acres of natural forest/hills with bison, mountain goat, bighorn sheep, elk, moose, blacktail deer.