Of the six facilities listed, I have visited them all, but I haven't been to Toledo for ten years or so. I would agree with okapikpr about excluding the Wilds ... apples and oranges.
Both Cincinnati and Columbus rise to the top for me:
Cincinnati's highlights are the unusual members of the collection (notably the Sumatran rhinos, also the small cats, prosimians, crowned guenons, alcids). Unfortunately, in the past 10 years or so they've lost and/or gotten rid of some of their jewels, including zebra duiker, mountain tapir, Temminck's golden cat, bharal, and shoebill. The exhibits are largely well done - most are older grotto-type enclosures which are surprisingly "fresh" since they have mature vegetation in them. The "immersion" exhibits are very well done. The vegetation is fantastic (it IS a Botanical Garden), but the organization of the zoo is pretty haphazard and largely taxonomical (cat house, reptile house, wildlife canyon, bear grottoes, cat grottoes ...)
Columbus feels much larger, and is now set up (sort of) zoogeographically. The exhibits are largely new, but some still have that obvious "new exhibit smell" and have yet to grow into the finished product. The collection is large and varied, and in another decade I would expect Columbus to be the hands-down leader.
My overall impression of Cleveland was not that positive (see my review in the Cleveland thread), and it will take them a lot of work to get up to speed with the two zoos mentioned above.
My recollections of Toledo are rather vague, and the African plains and Polar expansions had not yet been built when I visited. The Hippoquarium(TM) was fantastic - so much so that it has set the course for almost all new hippo exhibits being built! Like Cincinnati, I found the layout higgledy-piggledy.
Akron is a very unusual little zoo (not many places keep Himalayan tahr and sika in glass-fronted enclosures!), but I was disappointed with their use of space. In Legends of the Wild I was surprised at the smallness of the (naturalistic) enclosures for the jaguars and snow leopards, and the ungulate enclosures are also a bit "tight" (especially the alpaca). The bat building (also with lorises and snakes) was nicely done. Tiger Valley was my favourite section; the cat enclosures were quite densely vegetated, although the sizes of the enclosures still left something to be desired. The sun bears appeared quite comfortable in an exhibit with many options. I especially liked the red panda/muntjac/crane area - again, not huge enclosures, but with varied plants and interesting use of vertical space for the pandas (a partially terraced enclosure with a climbing structure and fresh bamboo). Komodo Kindgom was a very commercial structure - cafeteria and seating, with the Komodo dragon, Chinese alligator, and Galapagos tortoise exhibits tucked away to the side, almost as an "add on". Compared to the rest of the zoo, Wild Prairie was a lot of walking for very little reward. The enclosures here are mostly older, with extensive use of concrete to form moats and pools. I found myself wandering through this part wondering if I was on the right path since after the exhibits, you get "left" in a garden with several options and little direction to the rest of the exhibits.