I should probably pick Detective Conan, which is currently on it’s 908th episode and has been in production January 8th, 1996. I use to watch this series but I lost interest years ago. I won’t pick, One Piece because while it’s long, I think it’s as long as it needs to be. One Piece has a lot of story left.
My pick for the anime that is too long is, Dragon Ball. This series was started in 1986, the first series Dragon Ball lasted 153 episodes. Dragon Ball Z reached 291 in the late 90’s, then there was the terrible Dragon Ball GT which is thankfully the shortest with 64 episodes. If you could get through that series you probably had enough and was grateful that Dragon Ball was over. However, in 2009 Dragon Ball Kai was released and featured 159 episodes.
Personally, I didn’t watch most of the latest series. I got too repetitive find ways for saiyans to get get stronger and stronger. On some level this has always been true of the series. However, I feel that after the Cell saga Dragon Ball really went to far with the transformations. Super Saiyan 3 was cool looking but it didn’t really feel all that different. Weirdly it seemed that SS2 was more important than SS3. Villains kept getting ridiculously stronger and the particular formula was Goku quickly gets stronger, Vegeta then follows and nobody else does. That said the final arc of Dragon Ball Super did try to provide a opportunity for less powerful characters like Krillin.
Of course my biggest grip had to be the downgrade of Gohan. During the Cell Saga Gohan had surpassed everyone being the first to reach SS2 at age 9. He later put school work a head of his training (which maybe due to only being half Saiyan). Gohan maintain a great potential, Vegeta suggested that Gohan had the “potential exceeded both Goku’s and his own.” While I think that the Dragon Ball series was too long I did enjoy much of it (not GT) and the tournament arc at the end of Super was great.