Snowleopard has his rankings somewhere and he has reviews of pretty much all the zoos in his road trip threads. There's also the thread where people rank the favorite zoos they've visited.
I'm not going to give a long response in terms of why I rank the Top 25 USA zoos as I do, but of course there are many factors. The size of the animal collection, the quality of the exhibits, the history of the establishment, the overall "feel" of a great zoo, etc, etc, etc. There are countless reasons why a zoo is placed above another in the mind of any enthusiast.
Zoo rankings are mainly subjective but here would be my tentative list:
1- San Diego Zoo (2017 x 2, 2011 x 2, 2008, 2006) = 6 visits
2- Omaha’s Henry Doorly Zoo (2012, 2008) = 2 visits
3- Saint Louis Zoo (2014, 2010) = 2 visits
4- Columbus Zoo (2008)
5- Bronx Zoo (2008)
6- Sedgwick County Zoo (2010)
7- North Carolina Zoo (2010)
8- Denver Zoo (2012, 2006) = 2 visits
9- Woodland Park Zoo (2017 - 1994) = multiple visits
10- San Diego Zoo Safari Park (2017, 2011, 2006) = 3 visits
11- Zoo Miami (2008)
12- Dallas Zoo (2015, 2010) = 2 visits
13- Houston Zoo (2015, 2010) = 2 visits
14- Minnesota Zoo (2014, 2008) = 2 visits
15- Oklahoma City Zoo (2008)
16- Detroit Zoo (2008)
17- Kansas City Zoo (2010)
18- Brookfield Zoo (2008)
19- National Zoo (2008)
20- Jacksonville Zoo (2008)
21- Oregon Zoo (2016, 2011, 2010, 2008, 2007, 2005) = 6 visits
22- Phoenix Zoo (2011, 2008) = 2 visits
23- Arizona-Sonora Desert Museum (2011, 2008) = 2 visits
24- Tampa’s Lowry Park Zoo (2012)
25- Fort Worth Zoo (2015, 2008) = 2 visits
* I've included the year(s) of my visits
My personal U.S.A. total: 325 zoos/aquariums (248 zoos and 77 aquariums)
All-time zoo total: 372 (worldwide)