Marian, I love Woodland Park Zoo. It is a very relaxing place to visit. Though I haven't been there since I was a child, I have very fond memeories of the Catskills Game Farm in New York.
The top of my list of zoos is the Tucson Senoran Desert Museum in Tucson, Arizona. Not only does this place try to make all of the animals enclosures "natural", you can see so many wild animals while walking the paths. This zoo is in the middle of the desert with no fences surrounding it.