I found the Sam & Frodo scenes in Return of the King to grind...on and on... the dark tunnels, the blasted landscapes, the orcs marching hither and yon, gollum-gollum, climb, fall, more blasted landscapes, more orcs, etc. The first time through that was worse than the second, though. I've read LOTR twice. (Three times? No, I think just twice.)
But the worst series for me to finish (so far) was The Dragonlance Chronicles. I was expecting this series to be so good because of the high praise for it on Amazon.com. But when I actually read it, I found much to irritate me (like the paper-thin characters), much that was goofy about the sense of the passage of time (e.g., those impromptu conversations the heroes kept having while there were enemies running about), and there was often no clear viewpoint character - the viewpoint kept bouncing around, just who's supposed to be telling this story? Anyway, I draaaaaaged myself all the way through the DL Chronicles, and, eventually, it ended.
Jerry Abbott