Was Luffy able to activate/experience Kenbunshoku Haki in the War of the Best?
From the One Piece Wikia article on Kenbunshoku Haki:


  The first time it was seen being used and referred to as Haki was
  during Luffy's battle on Amazon Lily with the Boa Sisters. Later,
  Rayleigh explained Haki to Luffy and confirmed that "Mantra" is the
  Skypiean name for the ability.