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
Terror in Resonance
Tokyo Ghoul (and maybe Tokyo Ghoul :re though it was rushed)
Darker than Black (especially the first season)
Ghost in the Shell
The Breaker and Breaker New Waves
Hmm, there must be a few... Lemme check my MAL. Also, I'm only gonna list the type of animes I most often see on the forum, but there are a couple more.
Death Note (really good plot) Ergo Proxy (I really liked the vibe) Psycho-Pass (mostly the first season) Yuu☆Yuu☆Hakusho(perfect in every way imo) Zankyou no Terror (except for that annoying girl, it was pretty much flawless)
Edit: oh, juts now noticed the long running series part. Dunno how I missed it. Anywaaaaay.