Is ONE PIECE episode 308 filler?
Canon No, episode 308 of ONE PIECE is canon.
#308
Wait for Luffy! Mortal Combat on the Bridge of Hesitation!
ルフィを待て！ためらいの橋の死闘！
Aired 6 May 2007
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.