I’ve read the 20.5 Aubrey-Maturin novels something like 3 times through.
I’m also thoroughly enjoying the Song of Ice and Fire novels by George R. R. Martin (otherwise known as Game of Thrones), and The Expanse series by James S. A. Corey. I can’t imagine that I’ll not finish them- they’d have to drop WAY off for me to quit.
I also liked the Dire Earth Cycle by Jason Hough, and am about to start the second trilogy in that world that he’s written.
The first Hunger Games book of the series literally made me cry. I continued on with the rest of the series blindly believing that it couldn’t get any worse but of course it did including just getting blindsided in a few paragraphs near the end destroying what I thought was going to be the point of the series. They call that “Young Adult” fiction? The whole series is brutal, twisted and depressing. I love it in a way but it isn’t for kids.
All of the original Hardy Boys, from The Tower Treasure to The Firebird Rocket. I did read a few of the paperbacks that came later, but they didn’t have the same magic, even though I’m well aware the original series was not only ghostwritten but was also revised from the original versions. It was always weird finding an original Hardy Boys at the library or a bookstore.
The Dresden Files.
Harry Potter.
The Sime/Gen series by Jacqueline Lichtenberg and Jean Lorrah.
This for me, too. I’ve almost abandoned it a time or two, but keep going for the hilariously eccentric secondary character, Lucian.
Also,
Louise Penny’s Three Pines mysteries just keep getting better and better
John Le Carre’s Smiley novels
Kate Atkinson’s Jackson Brodie books
Armistead Maupin’s Tales of the City series
Tana French’s Dublin Murder Squad
and finally, Adrian McKinty’s Sean Duffy novels for the great music references, the vodka gimlets and all those mercury tilt bombs.
Another recent series comes to mind–Karl Schroeder’s Ventus (5 novels.) Here is the wiki description of the first book in the series.
It happens when a writer discovers that he has nothing to write, but the readers demand something to read. Then is when you get things like Steinbeck’s “Travels with Charlie” and Cormac McCarthy’s “The Road”. Or when John Patterson starts hiring graduate students to write his novels for him.
:dubious: “The Road” has nothing to say? Dude, I get it’s not your cuppa, but that’s a ridiculous statement.
Wrong thread?
Me too, and I’ll second the God help me. I did read the prequels and they actually weren’t that bad compared to some of the others in the series. I even read Kingdom Come, which was an absolute dreck of a novel tacked on the end of the series to milk it out even further. I didn’t read the YA novels either, I had to draw the line somewhere.
I enjoyed all 11 novels of the Malazan Book of the Fallen series. I’m behind on the side novels though.
Sanderson’s Mistborn trilogy was really good too.
In the YA segment, I enjoyed Rick Riordan’s Percy Jackson and Kane Chronicles series. They’re a fun retelling of ancient Greek and Egyptian mythology, respectively, in a modern setting. His characters have a fantastic snarky voice that you can practically hear from every page.
Richard Roberts’ Please Don’t Tell My Parents series is good, too. I have to admit that the sequels haven’t quite reached the height of the first book, which has a fantastic premise and a nearly perfect execution, but I’ve still liked the rest of the books. I’m really excited about the upcoming finale, which has another great premise that I hope will focus his writing better than the last two. They kind of meandered because they didn’t have an overarching plot to focus on.
When I was a kid, I loved Edgar Rice Burroughs, and read all of the Mars stories (even the shoddy later ones), and all of the Venus stories (which were fairly consistent).
I haven’t read all of the Tarzans, but I have read all of the ones that were adapted into episodes of the Filmation cartoon. Does that count for anything? 
I have read all of Ian Fleming’s James Bond stories, and all of Donald Hamilton’s Matt Helm stories. Of course, Bond is a cooler character than Helm, but I think Hamilton was a better writer than Fleming. (Or at least had better editors. Fleming’s books varied widely in quality. Hamilton was more consistent.)
I read Michael Moorcock’s Elric series, as it existed when I was in junior high school. When I was in college, he started publishing new stories. I meant to read them, but never got around to it.
I believe I have read all of Robert E. Howard’s Conan, Kull, Solomon Kane, and Bran Mak Morn stories. And deCamp and Carter’s pastiches.
I read all of Roger Zelazny’s Chronicles of Amber. I was really angry about the way he wrapped up the last one. Then he died, and I realized that he was staring the reaper in the eye, and trying to make the best of a bad situation.
I read The Silmarillion, The Hobbit, and The Lord of the Rings. Haven’t gotten around to HOMES.
I’ve been reading series ever since I got The Chronicles of Narnia boxed set in 5th grade.
I’ve said that Laura Ingalls Wilder was my best friend growing up and I read all her books by 9th grade.
Since I mostly read mysteries and I’m often broke, I don’t buy books. But I own all of Jan Karon’s Mitford series, and now that I have a decent job I may acquire the Father Timothy books as well.
I managed to read all of Anne McCaffrey’s Dragon series even after they got too confusing with too many characters and too much time travel.
Since mystery authors write in series, there’s many still going. There’s also way too many to list here.
I’ve done Percy Jackson, Harry Potter, Lemony Snicket, the Dark Tower, all but the last 5 or so of Discworld, and unfortunately Piers Anthony’s Incarnations of Immortality. Other stuff I recall finishing:
The Tomorrow series - seven books, starting with Tomorrow, When The War Began. Never felt it dragged, although our heroes’ escapes were getting more and more implausible by the end.
Escape from Furnace - five books, starting with Lockdown. It did get absurd toward the end, but the action comes at you fast, so you don’t really have time to think about it.
Demonata - ten books, starting with Lord Loss. The books are short, quick reads. I think what kept me going was, it’s a pretty gory series for one aimed at grade 8 or so. The same guy who wrote The Vampire’s Assistant. Which reminds me:
Cirque du Freak - twelve books, starting with A Living Nightmare. Decent books, but this series and the two above follow a very similar pattern of being drawn into paranormal stuff -> power escalation -> basically a god.
Skulduggery Pleasant - nine books and a few novellas, starting with Skulduggery Pleasant. Can’t recommend this series highly enough. If you liked Harry Potter, you need to give this series a try.
I will admit that I have read the entirety of L. Ron Hubbard’s Mission Earth series. It was at a time when I had an insatiable appetite for all things SciFi. It was the early '90s, so I had no idea what Scientology was. I would never attempt to read a 10 volume 4000 page series now days.