First up - from the Bronze Age: Marvel's Werewolf By Night
What happened was I was a huge fan of the Mike Ploog era of this series. I also have a soft spot for horror/adventure stories that feature characters that don't fall in the standard mold. So, initially, I found a lot to like with WBN. Ploog's stylistic storytelling approach was just a huge bonus.
Unfortunately, after Ploog left, I think the series started to stray from its creepy core. By the time Moon Knight showed up, I was pretty bored with the series and was just buying it out of habit.
But I hung in till the bitter end...
I took a break from most mainstream comics around this time, delving into more of the independent publishers that were cropping up (Comico, First and Dark Horse being the main ones I was reading at this time.) The one mainstream DC series I managed to follow during this time period was the Giffen/Levitz Legion of Superheroes. I dropped off a bit around the Late 80's (it was probably the continuity mess caused by Crisis on Infinite Earths which caused me to lose interest in the series.) However, when I heard about the premise of the 5 Years Later story line I jumped back in the game. It was just the thing for a lapsed DC fan like me.
Those first few years of the 5YL stories were amazing. I didn't always understand some of the references or know who the characters were, but my experience with the more mature stories in Independent comics had taught me not to worry about such things.
Alas, after a few years of stories where you reveal just how much everything has changed, you sort of run out of stuff to show changed, and end up with generic superhero stories. And the solution to this problem (introducing the SW6 batch of LSH Silver Age duplicates) was not way to solve the problem, imo.
Finally, the most recent series I just couldn't stop buying was the 2000's era JLA. Like most comic fandom, I loved the Grant Morrison and Howard Porter's run on the book:
...but the series quickly lost steam after Morrison left (despite a few good stories by other writers.) I think the problem here was, Morrison was telling huge, fantastic stories that the JLA happened to appear in. Other writers tried to tell big stories with the JLA. They sort of begin the project with a handicap off the bat so to speak. I remember barely being able to get through some of the later storylines (Busiek's Crime Syndicate storyline being a real chore for me iirc.) By the time Byrne and Clairemont had a shot on the series, I was well and done. I remember liking their arc on principle but not on merit.
So, what about you? Have there been any series you just kept buying long after you stopped enjoying them?