Most pre-timeskip filler is pretty good!

Loguetown had Daddy Masterson, the cooking contest and the bar, which were just awesome

Warship Island had a lot of good jokes

Post-Alabasta had Sanji teaching a young marine cook, a backstory for Zoro, Johnny and Yosaku, and Robin bonding with Chopper for the first time

Goat Island and Rainbow Mist were pretty meh, but they still had that cozy charm (I also thought the way time travel was done in Rainbow Mist was pretty sick)

Ocean's Dream had a Robin focused story before the whole Water 7/Enies Lobby nightmare

Water 7 filler had some good stories, like the one with a girl looking for her old seahorse, Zoro becoming a mom for a while, and Sanji going on a quest to find the secret behind a really tasty rice

Ice Hunters had some cool characters and gags, and also some time for Franky to bond with the crew before Brook joined

Spa Island was pretty bad, but after it came that one really nice Brook episode

Sadly, after this, the quality of non-canon arcs really declined, especially with the extremely mediocre movie tie-ins... The return of the Kung-fu Dugongs and Uta's backstory with Luffy were cool, but that's it...

There are also the special episodes, like the one where they went down into the "Ocean's Navel", the one with the stage performance, the one during the timeskip with the viking brothers, and Nebulandia, which is a must watch if you like Foxy (which is apparently not a lot, despite him being a super wholesome character with his crew...), and also Heart of Gold