This is a very hard question and i don't think we will get a
definitive answer. The first tune to use the term reggae was 'The
Maytals' with 'Do the Reggay' in 1968.
Among the contenders for the first reggae tune are:
Larry & Alvin - Nanny Goat
The Beltones - No More Heartaches
Lynford Anderson - Pop A Top
Lee Perry - People Funny Boy
As I say this is very debatable but those four seem the main
contenders.