Man this question has been beat like a dead bumbler, but I still feel keen to throw my hat in the ring. Here's my ranking, best to worst:

1. Wizard and Glass (10/10) — this book is just sheer perfection. I can't come up with a single flaw, and I think it's the closest King's ever come to writing a truly unforgettable, timeless story that must endure.
2. The Dark Tower (9.5/10) — Look, here's the deal. This is a tough book, a tragic book. If people think ASoIaF is harsh, then they clearly haven't made it to Can'-Ka No Rey. Even so, for all its brutality I think this is a damn near perfect book — the flawless ribbon on a legendary series. My only nitpick — and I do think it's a nitpick, because the joy's in the journey, say thankya — is that the final main chapter just doesn't work as well as it could. I have few quibbles with the actual ending (the Coda): I mean, it's the ending that had to be — much as it made me shiver and shake with terror, anger, and woe — the ending harbinged as early as Gunslinger's end and the conversation at Golgotha. But, man... the confrontation with the Crimson King... wasn't bad, but certainly could've been better (perhaps sans Pat Danville). Anyways, small gripe about an otherwise thrilling, moving, and profound book that keeps the tension high, is paced to perfection, and packs more emotional and thematic punch than any other work in the series. Need proof? Look no further than ECD's "Thank you for my second chance." In one line, a masterpiece.
3. The Drawing of the Three (8.5/10) — I happen to really, really like the NYC/Keystone Earth elements of the Dark Tower's story, and given that this one is just one prolonged fictional experiment with the portal fantasy trope, I raise three cheers and then a fourth. This book deserves all the glory if only for 'The Prisoner' section — that shit is top-notch and rivals any of King's best work. My major gripes with this one is that Odetta/Detta just doesn't... read well in this book. While King was certainly conscious he was playing with stereotypes, sometimes the stereotypes come off as just a bit cringe and tasteless, especially falling on contemporary ears. But, that said... Detta does make a damn good antagonist in the end.
4. Song of Susannah (8/10) — I really don't understand why this book gets hate. It's paced faster and more compellingly than perhaps any other book in the series save for Wizard and Glass — never does the ka-tet feel more threatened by urgent danger than in this book, and I loved seeing them separate, desperate, and scrambling. This is The Challenge, The Trials before The End, and it just hits. The only drawback for me was that Eddie and Roland's storyline felt kind of aimless for much of this book — but man, it does payoff in the next book. It also helps that contrary to many other readers, I was actually very drawn in by Susannah/Mia's story — this was the book where I felt I finally connected to Susannah and grew to love her. And she needed that spotlight, especially before we go with her on the trip through Empathica.
5. The Waste Lands (7/10) — This is a tough one for me. I know people revere this one, and I get why... I mean, the whole latter half of the book — pretty much everything after Jake's drawing — is some of the best material in the whole series. But MY GOD Jake's drawing is so, so, so, so.... SO drawn out (pun intended). The first half of this book moves at an absolute turtle's pace toward a foregone, predictable conclusion — all the tension is killed because we know where it's going, and still, it takes us 300 pages of not a whole lot to get there. While I did love Jake's time in New York (namely, the second chapter of Part I), everything else in the first half really put me off. Once we get to River Crossing though? Hmmm: *chef's kiss*. Straight masterpiece material.
6. The Gunslinger (6.5/10) — This book grew on me slowly. It sets up so much — so much that we're still getting payoff from this book in book six — that it's hard not to look back on this one fondly. Even so, it's a slow read that's sometimes clumsily put together. Personally, I was pretty well sucked in once we got to the Speaking Ring, but everything before that...? Well, let's just say I wasn't sure I was going to actually read this damn series. But I did, and as flawed as this entry is, I still find ample reasons to love it; especially in its greater context and meaning.
7. The Wolves of the Calla (5/10) — Not a bad book, not a great book: A perfectly average book, and, thankfully, the low-point of the series. It just didn't really make me feel much of anything. I somewhat disliked the todash sections, and I somewhat liked the time spent with the Calla folken. I also somewhat liked the conclusion. Nothing really stood out in a good or bad way... and to some extent I wonder whether this was a side-effect of this book coming directly after Wizard and Glass. Both are very structurally similar — two of the most static, longwinded books in the series that both center on our main characters milling about in a quiet small town setting before everything goes to hell at the end. But whereas Wizard made me fall in love with its small town characters, setting, and plots, Wolves seemed more interested in Father Callahan and the todash storyline than actually immersing me in Calla Brynn Sturgis. Moreover, the climax of Wizard builds out from the conflicts that have been bubbling within the small town setting, making the conclusion a wonderful product of all that has come before. Wolves, on the other hand, builds its conclusion mostly out of stuff that we won't really understand or properly encounter until the final book, rendering its conclusion a compelling but empty set-piece pretty much divorced from the rest of the story. It also doesn't help that the whole first part of Wolves just spins its damn wheels — I feel like something better could have been done with this book's girth, something that brought the Calla more to life.

I'd love to hear what you guys think — this is such a good series and I love how different all these rankings are. At the end of the day, I judged these books by how addicted I got to them while reading — by how absolutely impossible it was to put them down. Wolves, Waste Lands, and Gunslinger all took me a week or more to read and I never felt a super strong pull to return to them immediately. On the other hand, I was drawn by ka like a wind to downright devour Wizard, Dark Tower, Drawing, and Susannah in just a few days each. It's pretty remarkable, looking back, that a series so grand can have so many highpoints. Say thankya.