I went through a lot of players in the WAR database and I found all the seasons of +11 WAR since 1955. So without further ado here are the best seasons since 1955 as measured by WAR:

1956: Mantle 13 WAR

2001: Bonds 12.8 WAR

1957: Mantle 12.5 WAR

2002: Bonds 12.3 WAR

2004: Bonds 12.2 WAR

1967: Yaz 12.1 WAR

1975: J. Morgan 11.8 WAR

1961: Mantle 11.7 WAR

1982: Yount 11.4 WAR

2001: Sosa 11.3 WAR

1977: Carew 11.2 WAR

2003: Pujols 11.1 WAR

2000: A-Rod 11 WAR