Is ONE PIECE episode 266 filler?
Canon No, episode 266 of ONE PIECE is canon.
#266
Battle Against Giants! Open the Second Gate!
巨人族との攻防！第２の門を開けろ！
Aired 11 Jun 2006
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.