Non canon arc so good, People thinks it's Canon.

1.Zanpakuto arc from Bleach was considered to be the best filler arc by it's fans due to the Canon design of the zanpakuto but also great story.

2.G-8 arc from One Piece. The infamous filler non canon G-8 arc airing before the long ring long island arc (It was terrible on the anime but great on the manga)