Is ONE PIECE episode 638 filler?
Canon No, episode 638 of ONE PIECE is canon.
#638
A Deadly Blow! The Astonishing King Punch!
一撃必殺！驚異のキング・パンチ
Aired 30 Mar 2014
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.