I love going to the Vilas zoo, although when the weather is nice on a weekend, the sheer number of children can be overwhelming.
Pros: free, free parking (you can find street parking if lot is full), good selection of animals, open 7 days a week.
Cons: Some of the animals need bigger exhibits. In particular, I noticed that the giraffes were in a really small indoor space in winter, and one of them was licking the wall incessantly which doesn't seem like a good sign. There is lack of good maps--the main map in the center of the zoo doesn't show anything just their future plans for the zoo.
PS--make sure you see the red panda. It's in a totally obscure part of the zoo, in the area near the carousel, all the way in the back.