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.