Although I havn't been there yet,the Giants of the Savannah most entrigues me. Hope to visit Dallas summer of 2011.
I chose Lied Jungle, for now. I hope when Asian Tropic at Denver Zoo makes it on a list like this when it opens in a couple.
I've never been to Detroit Zoo, but after hearing about Arctic Ring of Life and hearing amazing things, it's an exhibit I'd love to see! Maybe sometime this winter.
I've visited all 8 of the exhibit complexes within the past few years and I voted for Bronx Zoo's "Congo Gorilla Forest". Other top-notch nominees could have been "River's Edge" at Saint Louis Zoo, "Oklahoma Trails" at Oklahoma City Zoo and "Russia's Grizzly Coast" at Minnesota Zoo.
I went with Lied Jungle because despite some issues in exhibit size, it is by far the most exciting exhibit I have visited from the visitors perspective. The next closest are likely their other two major immersion exhibit complexes. CGF is great as well and better for the animals on average, although much of their indoor section is nothing to write home about. It's not nearly as exciting as Lied though imo and it's not much of an exhibit at all for 4 or so months a year as I just experienced.