Out in the wild, most definitely. Although I suppose technically they are wild animals, I don't consider animals in captivity to be "wildlife." It does make them a lot easier to photograph, though. My favorite place to shoot wildlife is the Klamath Basin National Wildlife Refuge, about an hour and a half drive from where I live, and much closer than the nearest zoo! :-)
Here in Kingston there's a conservation area with walking trails that's populated with deer and many varieties of birds, owls etc.
Soon to move to Cape Breton Island, Nova Scotia and will offer Bald Eagle, Bear, Moose and more!
In the wild is really the most exciting, and the most rewarding for me....you just never know what you'll find!
Both have their own plus and minus points. I live in Oregon and have many opportunities to image in the wild, but every now and then the zoo is visited for things like trying new equipment and training, the yearly butterfly house, and a big 5 image every now and then. I prefer the wild, but one thing you've got to remember - be safe. I photo snakes and it is very easy to forget/disrespect a wild animal and then both the human and the animal will suffer. Also when you are miles from your car and something happens, say like a "killer" bee hive, things can get a little less "happy".
When I lived in Montana I used to drive over to the National Bison Range in Moise. Hardly for the Bison. I loved to photograph the Rocky Mountain Bighorns. I never had to deal with the selfishness of typical tourist since the range was not as popular as say Yellowstone during the summer. A photographer who has discipline and respect for nature could create some nice images without spooking the subjects.
Next favorite was Glacier National Park. 2 1/2 hours from my driveway to the west entrance. Bee line to Logan's Pass, hike for 2 miles in Grizz country to a very accommodating herd of Rocky Mtn. Goats. I've had zen moments there that I am unable to describe in words.
The black bears in the Smoky Mountains are a hoot. One morning at 5 am the road into Cades was closed to automobile traffic so we had to hike in to photograph Sparks La for sunrise. Has 6 bear encounters in 30 minutes. I actually don't do wildlife anymore..but if I did a hike into Cades between 4 and 5 am the local bears will have a human sighting.
I live near a lake in Florida - it's great for water birds - although I'm not that fond of ducks, but Herons, Egrets and others. This should be a really good year as the lake is 'full' again. When I was first starting to take my photography a little more seriously - I ruined a beautiful set of snowy egret shots by down sizing them for Flickr: http://www.flickr.com/photos/FlaShots/
I agree with Eric, any lake in Florida is good but I'm partial to wetlands like Payne's Prairie, St. Marks National Wildlife Refuge and the Everglades. They are target rich environments for birds and other wildlife.
My nearly single favorite spot for birds and mammals is Cave Creek in Coronado National Forest, including Cave Creek Ranch, in Portal, AZ, just before the park entrance. You can see coati mundi and javalina and many species of birds including blue throat hummingbird, and other hummingbird species, Acorn woodpeckers are plentiful. The elegent trogon nests in the area. Deer walk through campsites in the campgrounds. I've even heard that a cougar walked through the campground host's campsite in sunny flats campground about 4 years ago.
If you broaden your scope to points west, as far west as madera canyon, you may find every species of US hummingbird in the US, except of course, the ruby throat including relatively rare ones like the white eared, and the lucifer.
During spring migration, Aransas TX is great for many species, and for whooping cranes.
Just as the best camera is the one in your hand, at the moment, the best location is the one that's near enough to visit. That's a subjective judgement, since I live in Atlanta, and am recommending locations in TX and AZ. Likewise, season, and the species you want to add to your portfolio are very important.
The two main places I photograph wildlife is in Rocky Mountain National Park and the Pawnee National Grasslands in Colorado. There is a wide variety of animals, birds, and insects for me to photograph between those two locations. I enjoy the challenge it takes to find them and get good pictures of them in their natural habitat. There is also a park in the city I live in that attracts lots of rare birds in the Spring and Summer.