Is ONE PIECE episode 773 filler?
Canon No, episode 773 of ONE PIECE is canon.
#773
The Nightmare Returns! The Invincible Jack's Fierce Attack!
悪夢再び 不死身のジャック強襲
Aired 22 Jan 2017
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.