The St. Louis Zoo is the best zoo I have ever visited! Not only do they have a wide variety of animals on display, but you never run into the problem of not being able to see an animal. Each "stage" is designed to seem as natural as possible, affording you the closest view without putting you in any immediate danger. The prairie dogs, hippos, penguins/puffins, and zebras were my particular favorites.
Families, couples, and individuals will all feel welcome. If you go during the week, you will feel less crowded. I have visited more than 10 times and have never been disappointed! In fact, I still can't get over the fact that it is free!!!