Where is the largest collection of baseball cards on display ?

According to this web site, the Jefferson R. Burdick Collection is the largest and most comprehensive collection of American trade cards ever assembled in the United States.

Until 1993 the cards were available for viewing at the Metrpolitan Museum of Art by appointment only. In July of that year, in response to the enthusiasm of young collectors, the Metropolitan installed 16 sets of baseball cards in special frames on the first floor of The American Wing.