What was the code name for Windows Vista?

The code name for Windows Vista was Longhorn.

Microsoft began developing Longhorn in May 2001, several months before Windows XP was released. It was initially planned as a relatively small successor to XP, positioned between XP—whose code name was Whistler—and a future major release called Blackcomb. The name Longhorn referred to a bar located between the Whistler and Blackcomb ski areas in British Columbia.

Longhorn's development expanded dramatically as Microsoft added technologies intended for Blackcomb, including major changes to search, graphics, communications, and storage. Delays and feature creep led Microsoft to reset the project in 2004, rebuilding it around the Windows Server 2003 code base. The product was renamed Windows Vista in 2005 and released to manufacturing on November 8, 2006.

The other options are also Microsoft code names, but for different projects: Memphis referred to Windows 98, Chicago to Windows 95, and Whistler to Windows XP.