The Building of the Year RIBA Stirling Prize is the most prestigious award for architecture in the UK. It goes to the architect for the building that has been most significant for the evolution of architecture in the year. Kevin McCloud visits each of the final six shortlisted buildings to find out their stories, meeting the people who use them as well as those who built them, and finding out about some of the challenges the architects faced turning their designs into reality. It's always a close-run competition and 2009 is no exception, with a dazzling array of top-quality buildings on the shortlist, including an elegant 18-storey office building in the historic heart of the City of London; a pioneering cancer care centre at Charing Cross Hospital, which treats the emotional and physical needs of patients; a stylish Modernist art museum in the Danish wilderness; a £500 million redevelopment of Liverpool city centre; and a new NHS health centre in Kentish Town, which represents the future of general healthcare architecture in this country. Plus Tom Dyckhoff, Deborah Saunt and Sarah Gaventa champion buildings that they feel should have made it onto the Stirling Prize shortlist, and Sarah looks at previous winners to see what qualities make a building a Stirling Prize winner.