Following your example, my favorites are The Stand, Salem's Lot, The Shining, Duma Key, The Drawing of the Three, The Wastelands.
My absolute favorite is Wizard and Glass. Perhaps the only volume in the Dark Tower to actually be a western romance. It has wonderful characters, comedy, bitter sweet tragedy, tender romance and amazing gunfights! It really has it all. That is your Saturday matinee cranked up to 11. Readers that wanted to see the continuation of the plot were disappointed, but as a standalone novel Wizard and Glass is exceptional.
It is a shame that same author would end up bitterly lecturing his readers in the final chapter of the final volume.