It’s chinjao because since the question says “Luffy fought” he fought chinjao in the colosseum during dressrosa. The reason why it isn’t Garp is because Luffy didn’t fought Garp but Garp just beats Luffy.

Actually, Luffy did clash Garp in marineford, Garp let Luffy beat himself to the ground. Since both did go on the offensive (though Garp gave up), I counted that as a very short fight. So I guess depends on your definition.