The Fairchild C-123 Provider
"A number of C-123s were configured as VIP transports, including General William Westmoreland's White Whale. The C-123 also gained notoriety for its use in "Operation Ranch Hand" defoliation operations in Vietnam.
The first C-123s to reach South Vietnam were part of the USAF's Special Aerial Spray Flight, as part of Operation Ranch Hand tasked with defoliating the jungle in order to deny rebels their traditional hiding places.[6] These aircraft began their operations at the end of 1961. Aircraft fitted with spraying equipment were given the U prefix as a role modifier, with the most common types being the UC-123B and the UC-123K. Aircraft configured for this use were the last to see military service, in the control of outbreaks of insect-borne disease."
Source: http://en.wikipedia.org/wiki/Fairchild_C-123_Provider