Thought I covered that with Bleach, my opinion is that generally speaking every anime has a point it really should end, some earlier then others.
Naruto, yeah it wasn't particularly good to start with, but ignoring the fillers and a good amount of the shit after the fillers it's not terrible (this isn't the same as saying it's good). From what little I've seen of it's new series, this title has managed to get even worse, which is practically a achievement in itself now considering where it started.
Bleach was alright around the start when it was all about killing hollows and shit. Soul Society dragged, but had some amusing characters and moments, make the pacing better and it may have even been enjoyable. After that it just start to go to shit and the core reason it was amusing for any amount of time is lost as it gets dragged out further and they get more retarded and powerful.
DBZ should have ended where the writer originally wanted it to end, hell, I'd prefer if it ended at Freeza, Namek's pacing was shit, but some of the fights (most notably the Genyu Force) were amusing.
One Peice I'm not sure about, I stopped on around the Skypia arc and don't have the will to try any further. I liked the idea and One Piece works if you don't take it seriously and accept it's just pirates dicking about... but yeah, shit dragged on and that arc was just tiring.
Berserk, the anime probably made a good choice ending where it did. Leaves you wanting more but it covered the best part of the manga. After the golden age it's still good for the most part, but it's really started to drag down in the later chapters and I'd be happier to see it all end.