I'd have to say the Lord of the Rings trilogy, in particular books 2, 3 and 5 (yes, for those of you who don't know, each book is actually broken into two separate books, technically making it a trilogy of six.) The parts that I have the best time reading are the second half of The Fellowship of the Ring (from the forming of the Fellowship to its breaking) the first half the Two Towers (which follows Aragorn, Legolas, Gimli, Gandalf and the people of Rohan in their struggles against Saruman) and the first half of Return of the King (which follows the Gandalf and Pippin in Gondor, Aragorn braving the paths of the Dead with the Grey Company, Theoden, Eomer, Eowyn and Merry and the marshalling of the forces of Rohan, the subsequent battle of all upon the Pelennor Fields and the riding from Gondor to challenge Sauron at the Black Gate.)
The other three parts are good in their own way, but I feel like the first half of Fellowship of the Ring takes too long to get going and stops too much, while the second half of The Two Towers has a huge amount of focus on Frodo, whom I find a far less engaging character, and the second half of The Return of the King has the main climax of the story, which is good, then another hundred pages of everyone saying goodbye to each other.
I'm also a huge fan of the Harry Potter series, though the first four books are the only ones that I can readily read. I find that so much went so horribly wrong in Order of the Phoenix that I can barely stomach it, while I feel most of the magic was long gone by Half Blood Prince and Deathly Hallows (and was non-existent by the time Cursed Child came along!)