Shaman King! It's not long (64 ep.), but I have really fond memories of it, even though I have only seen the series once. Definitely something worth checking out it... if you are not familiar with the anime already.
its usually the shorter ones. series that are long as **** like shonen's will eventually dip in creativity at the volume they release material, its inevitable
but i think Samurai Champloo is perfect but its not a lot of episodes. Death Note is close but it kind of slips a bit after L dies, Code Geass is slightly too long but close. Attack on Titan was close for a while there but it started having boring sections as well