According to the Guinness World Records, the world's largest weather vane is located in Jerez, Spain.
According to the explore north website a challenger for the title of world's largest weather vane is located in Whitehorse, Yukon. The weather vane is a retired Douglas DC-3 atop a swiveling support. Located beside the Whitehorse airport the weather vane is used mainly by pilots to determine wind direction. This weather vane only requires a 5 km/hour wind to rotate.
Other claimants to the title are located in:
References
World's largest weather vane Wikipedia(Text) CC BY-SA