Is ONE PIECE episode 666 filler?
Canon No, episode 666 of ONE PIECE is canon.
#666
The End of the Match?! A Surprising Result of Block D!
勝者決定！？Ｄブロック衝撃の結末
Aired 19 Oct 2014
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.