I've visited 520 zoos (413 zoos and 107 aquariums) in my life, and that includes 372 zoos (286 zoos and 86 aquariums) just in the United States. I know that those numbers are totally bonkers, but it's all true and detailed over the past dozen years on ZooChat.
Undoubtedly, the worst state in the U.S. for zoos is Wisconsin, but let us first look at a few other contenders that have been mentioned in this thread.
California - Naturally, the two big San Diego parks are regarded as world-class zoos and I've visited more than 60 of the 70 or so 'zoos' in California. There are many top-class facilities, but also a whole bunch that are quite poor. America's Teaching Zoo, Applegate, California Raptor Center, Folsom City, Forever Wild, Free Flight, Project Survival's Cat Haven, Reptile Zoo (ghastly!), Roundhouse Aquarium and Santa Monica Pier Aquarium are 10 establishments that I've personally visited and they are all rundown or old or with shoddy animal exhibits. There's also other places such as Hesperia Zoo, Lions Tigers & Bears, a SeaQuest Aquarium, Shambala Preserve and Wildlife Haven Ranch Sanctuary that make it 5 more that I have not visited...but all look rather dodgy. One could make the case that California has at least 15 mediocre to awful 'zoos', but there are so many very good ones in the state that it's a hotspot for zoo enthusiasts. In fact, even with 15 below-par menageries, California is arguably one of the very best states in the U.S. for zoos.
Florida - There is an overwhelming number of zoos and aquariums here, circa 125 or so, and it's mind-boggling to view some of the low-budget gator parks that proliferate in the state. Only around 17 of the zoos are AZA-accredited, which is alarming, but Florida still has a lot of top-notch facilities scattered everywhere.
Georgia - There's Zoo Atlanta, much improved in recent years, and one of the world's largest and best aquariums, but around 20 other places that I've never visited and don't really feel an urge to see!
There are other states that aren't very good for zoo fans, such as Arkansas and Idaho, but there are too few zoos there to actively complain about. Texas is a state where I've visited 45 out of the approximately 50 or so zoos, and there are a number of duds, but at the same time I could make the case that Texas is the #1 state in America for zoos. It's got some really fantastic places and in fact my co-author and I included exactly a dozen Texas zoos (one less than California's 13) in our book America's Top 100 Zoos & Aquariums.
Wisconsin - This state is so bad for zoos that after my 2018 road trip I immediately started looking at flights to visit the truly great zoos of Western Europe. Ha! It's true that Milwaukee County has its fans, but so much of the zoo was built 60 years ago. Henry Vilas is a very nice, free zoo that I'd recommend, but there are tons of awful duds on the list below.
Here are the 28 zoos of Wisconsin that I've been to:
Alligator Alley (Wisconsin Dells)
Animal Gardens Petting Zoo (Delavan)
Animal Haven Zoo (Weyauwega)
Bay Beach Wildlife Sanctuary (Green Bay)
Bear Den Zoo (Waterford)
Beaver Springs Park Aquarium (Wisconsin Dells)
Bruemmer Park Zoo (Kewaunee)
Doc’s Zoo (Bonduel)
Glacier Ridge Animal Farm (Van Dyne)
Henry Vilas Zoo (Madison)
International Crane Foundation (Baraboo)
Irvine Park & Zoo (Chippewa Falls)
Jo-Don Farms (Franksville)
Lincoln Park Zoo (Manitowoc)
MacKenzie Center (Poynette)
Menominee Park Zoo (Oshkosh)
Milwaukee County Zoo (Milwaukee)
NEW (Northeastern Wisconsin) Zoo (Green Bay)
Ochsner Park Zoo (Baraboo)
Racine Zoo (Racine)
Shalom Wildlife Zoo (West Bend)
Special Memories Zoo (Greenville)
Timbavati Wildlife Park (Wisconsin Dells)
Wilderness Walk Zoo (Hayward)
Wildwood Wildlife Park (Minocqua)
Wildwood Zoo (Marshfield)
Wisconsin Big Cat Rescue (Rock Springs)
Wisconsin Deer Park (Wisconsin Dells)