While One Piece does have filler arcs, it's usually after a long arc, and because the anime is catching up to the manga. One Piece has the lowest amount of filler than any other anime. It's only 3% over 916 episodes.
Also, TPN is not filler free, but it's pretty close though, I read it weekly as well as watch the anime. There is nothing that is really 100% canon.
I'm near the end of Hunter x Hunter. I didn't know it was made by the same guy who made Yu Yu Hakusho until recently and when I found out I knew I had to check it out. I've been binge watching lol. It's awesome!
Also, TPN is not filler free, but it's pretty close though, I read it weekly as well as watch the anime. There is nothing that is really 100% canon.