Is ONE PIECE episode 551 filler?
Canon No, episode 551 of ONE PIECE is canon.
#551
The Battle is on! At Conchcorde Plaza!
決戦始まる ギョンコルド広場
Aired 10 Jun 2012
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.