This is a great thread and one that I've been following with a lot of interest as I've had a similar discussion with zoo friends in the past. The notion whether or not to include aquariums can alter the process, and that is an intriguing point as I can comprehend arguments from both sides. Anyway, the initial list of 5 nominees is almost correct but there could possibly be a change. Tennessee replacing New York would be my personal choice, and here are my thoughts:
Top 5 U.S. states for zoos:
#5 - Tennessee: Nashville is a brilliant little zoo, packed with many outstanding habitats and moden-day levels of exhibitry. Memphis is also a much loved zoo that has steadily improved over the years. Knoxville has some nice elements, and even tiny Chattanooga is worth a visit. Toss in the Tennessee Aquarium (possibly one of the 5 best in America) and this U.S. state makes it to #5 on my list.
#4 - California: San Diego Zoo and San Diego Zoo Safari Park are both top 10 material, but after that there are many good but not great establishments. Oakland is supposedly excellent but tiny (I have yet to visit), and there are fairly minor zoological parks like Fresno Chaffee, Santa Barbara, The Living Desert and Sacramento. I'm not a big fan of either San Francisco or Los Angeles, so after the two San Diegos there isn't a lot of outstanding stuff in the golden state. Aquariums are a totally different matter with several excellent ones, but only counting zoos I'd place California at #4.
#3 - Ohio: I love Columbus and think that it is a top 5 zoo, and Cleveland, Cincinnati and Toledo are all well worth visiting and each for different reasons. Cleveland has a fantastic rainforest and a large Australian section, Cincinnati arguably has the most diverse collection, and Toledo is a pleasant mix of old and new exhibitry. Toss in tiny Akron and The Wilds and based on the size of the state I can see why some folks place Ohio in the number one position. However, only Columbus is a top zoo for me while the rest are very good but not brilliant in any category.
#2 - Florida: There are some big guns in this large state, and I personally think that Zoo Miami would crack my top 10 and that both Jacksonville and Disney's Animal Kingdom (even with my hit-and-miss review of that billion-dollar establishment) are strong candidates for a position in the top 20 zoos in America. So right off the bat Florida beats out Tennessee, California and Ohio for top zoos. Then there is Busch Gardens, Lowry Park, and smaller places like Brevard, Naples and Palm Beach. I've even visited the private White Oak collection, but not even counting that place I'd have Florida in the #2 position.
#1 - Texas - This is America's second largest state (surpassed only by Alaska) and thus that aids things considerably. There isn't a zoo that would crack my top 10, but Dallas is awfully close and is by far and away the best that Texas has to offer. However, Cameron Park, Fort Worth, Houston, Caldwell, Gladys Porter, El Paso and San Antonio (in that exact order) are all worth visiting for one reason or another. I've personally toured all 8 of those zoos, and with Dallas standing out as the clear best, San Antonio as easily the worst, then there are 6 other zoos that are all of decent quality in the middle. No other state has the quantity of top-notch zoos, and to top it all off 2010 was an amazing year for Texas zoological parks. "Giants of the Savanna" at Dallas, "MOLA" at Fort Worth, "African Forest" at Houston, "Africa Live" at San Antonio and "Africa" at El Paso represents around $130 million in improvements, and so that state is my clear #1. There is only one truly great zoo, but the overall quality is very impressive.