The New York City Subway System
Massimo Vignelli's 1972 redesign:
Unlike Beck's London Underground system (which Vignelli had been influenced by) the above streamlined design was never truly accepted. In 1979 the map reverted back to its more literal design and all of the anterior content was again included.
Vignelli has since designed the map again in 2008 and launched a digital version in 2012. Read an article here about Vignelli's reaction to the map's numerous reiterations and a short explanation behind his more sophisticated design.
Other designs have been attempted as well. The Kick Map (below, center) came out in 2007 as a hybrid of Vignelli's version (right) and the more recent (left) designs:
And I couldn't leave out Veit Schuetz's illustration for the 2000 Cosmic Art Enterprise annual art exhibition: