Series
The Wheel of Time by Robert Jordan
The Dark Tower by Stephen King
The Dresden Files by Jim Butcher
The Underworld USA Trilogy by James Ellroy
The Lord of the Rings by J R R Tolkien
The Robot Series by Isaac Asimov
The Kingkiller Chronicle by Patrick Rothfuss
The Alex Stern Series by Leigh Bardugo
The Wrinkle in Time Quintet by Madelaine L'Engle
The Pendergast Series by Douglas Preston and Lincoln Child
The Adversary Cycle/Repairman Jack series by F. Paul Wilson
The first seven or so books are spectacular. True, then there's the slog. But by that point I was invested and the payoff was great by the time the series finished. Plus Jordan so loved his fans, that as he knew he was dying he still wanted the fans to have closure. He didn't lego ego trump his vision for wanting the story to be finished (looking at you Martin).
The Wheel of Time series is in its own category for me - books I will always love and be thankful for. They brought me many wonderful people in my life, including one very special person. And plus WOT is a good story.