The unforgettable An American Century of Photography was presented last summer at the Denver Art Museum, and the sprawling twentieth-century survey included some of the most important images ever produced. Curator and connoisseur Keith Davis made selections from the heavy-duty collection of Kansas City’s Hallmark Corporation, which has acquired famous photos by all of the superstars in the finest, rarest and most-sought-after print versions imaginable. Of all the fine photography shows presented this past year, none held a flashbulb to An American Century of Photography.