Well, most people will agree that the vast majority of fillers in anime are awful. It's very hard to make a filler arc good because you can't give any character development nor have the characters learn new moves/change in any way. Also, the creators of filler must be very careful not to disrupt the main plot of the story and not take any ideas the original author has planned to use from the beginning. But, there are a few that stand out and are worth watching, some even on par with the canon episodes. Does anyone here have a favorite filler episode/arc?
My favorite is the G8 arc of One Piece. It doesn't rely on Devil Fruits for the enemies, it develops the Marines into a sympathetic force, and it's hilarious while staying in the bounds of the OP universe (something most filler tends to forget about). The new characters are interesting and it helps to jump-start the series into a state of normality after spending so long with cloudy backgrounds. The best part is, it doesn't feel like a filler arc. The sense of immediacy is always there, and though you know the Straw Hats are going to go on as though nothing happened, the Marines always put up an interesting fight.