there are many other reasons which include, well-devised plots, exceptionally introduced characters, the longevity of hype and the character development, and introduction..
the conglomerate of many differ things essentially.. that can be the crew moments.. the interactions.. humour.., grandiose moments, tensions together,and themes en lessons
if you're only counting characters as in carrying the series.. then.. even tho i agree zoro has some of the sickest moments...
without him Impel down to MF stretch still stands as the best arc sequence in onepiece without zoro's involvement.. and the one at the center of it all was Monkey D Luffy..
soo Zoro, sanji, nami and folks being there doesn't matter its Oda's choice of emphasize on the consequent plot and the character playing the role in it.. and from my perspective LUFFY CARRIED AND STILL CARRIES THE FLIPIN MOMENTUM OF THE SERIES.. and through his venture.. had come some of the most titanic and monumental scenes and narratives in OP..
that made everyone or most atleast lable it as the best arcs as it stands.
in a nutshell..
Luffy carries onepiece..
but Oda is what give his character substance and that entails different factors in of itself