User blog comment:Xouzoura/What colour of haki do you think Luffy will decide to improve the most ?/@comment-98.103.41.35-20110302061624

While Luffy will obviously learn all 3 haki colors, I'd be willing to bet his natural one was the armaments, because he's the kind of person to jump right in without looking and let pure strength and endurance make up for his lack of observation. Also, he has a history of hitting harder than he should and taking stupid amounts of damage.