Is ONE PIECE episode 678 filler?
Canon No, episode 678 of ONE PIECE is canon.
#678
The Fire Fist Strikes! The Flare-Flare Fruit Power Returns!
火拳炸裂！復活メラメラの実の力
Aired 18 Jan 2015
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.