Which anime has the most number of filler episodes and the longest filler arc? Many animes have filler episodes and arcs. So this question has two parts:


Which anime has the most number of filler episodes? 
What is the length of the longest filler arc?
Percentage wise bleach has the most fillers, since it has a total of:

366 episodes

160 filler episodes


which means 43.4% of the anime are fillers.
naruto and naruto-shippuuden combined have

640 episodes
238 filler (1st season 89, 2nd season 149)

which means 37.2% of the anime are fillers.
one-piece has

682 episodes
97 filler episodes

which means only 14.2% of the anime are fillers.
But if you were to look at the sheer number of filler episodes, naruto has more fillers with 214 filler episodes.
And I believe the longest filler arc would be in rurouni-kenshin where the last 35 episodes were fillers.
Source: Own experience, Google, Filler Guide (webarchive)
EDIT

&quot;why in your estimation, none of the longer-running series beat out Bleach and Naruto under the definition of &quot;filler&quot; that others already provided above&quot; - seijitsu

Taking a few of your given long running samples such as sazae-san and nintama-rantarou makes the question quite simple to answer.
In the case of Nintama Rantarou non to barely non fillers have been aired. This does not mean it did not have fillers, because they certainly had. They were extras which as far as I know where only available on dvd. Furthermore they did not have a plot/story to follow, which makes the only deviation the moment the story would not be based on the manga, which in almost all episodes is the case. Also with the episode length given in this series 3 episodes would be the equivalent of 1 episode in naruto and the likes, which in this way easily out does this particular one.
In the case of Sazae-san, Sazae-san actually Did not have ANY filler episodes. Long running does not mean it needs fillers, there are several which never got a filler, even though they had a particularly long run time. Here follow a few

GE 999
Hoka Hoka Kazoku
gundam First
Sazae san
Kochi kame
Lupin III
the Super Robot Saga shows [ Maziger z Getta Robo and Grendizer]
Ninja Hitori kun