For
Cincinnati, it is without question the CAC (Contemporary Arts Center). It was the beginning of the downtown renaissance, it brought back (white) people to downtown, and spurred other developments including Fountain Square, the robot going up that's 660 feet, Underground Railroad Freedom Center, and The Banks. It was also Zaha Hadid's first American project.
http://farm1.static.flickr.com
For
Columbus, the Nationwide Arena. It literally bitchslapped downtown back into shape, brought over 5,000+ residential units, served as a model for arena districts around the country, helped "cap" I-670 connecting the Short North to downtown, revitalized the North Market district, brought movie theaters, restaurants, a new ballpark, condo towers, and *YES* pancakes!
http://www.sociology.ohio-state.edu/
For
Dayton, it would definately be the Schuster Center. A new condo tower and two new theaters designed by Cesar Pelli brought back (white) folks to downtown with starting off a new ballpark, canal district, Riverscape Park, new towers (CareSource, etc), and a (currently being) referbished Arcade, it was Dayton's much-needed shot in the arm.
http://www.batesline.com/archives/2008/06/20/
The main theater roof
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