monkey d. luffy is one of my highest kins !!! i think ive always had an attachment to luffy even before i got into one piece (yet i was never rlly open about it), but i wholeheartedly love him!! :D hes near and dear to my heart for various reasons but i think if one piece had a different protagonist it wouldnt be as fun of a watch as it is now. you cant spell 'king of the pirates' without luffy!
i think hes a very well-written character, even when the series spans for several decades oda does not forget that hes an airhead who is kind of really dumb but knows where to draw the lines! he grows drastically throughout the show and his development is very obvious as it progresses!! alot of people trash on one piece for its length and yes some of the pacing is super slow (dressrosa) but i think thats the whole charm of the show. you cant write a show about becoming the king of the pirates without it being insanely long! :)
above all that though, i think luffy is one of the greatest characters in anime of all time !! i could explain my reasoning for hours but to put it short, hes just a very likable doofus who grows to be an even more likable doofus :D