For me, each one was better than the last.
The Gunslinger was good, and it left you wanting more, but I definitely prefer the full Ka-Tet to just Roland and Jake.
The Drawing Of The Three introduces a new form of travelling between worlds, and Roland meets Eddie and Susannah, but I really wish Jack Mort was a bit more of a main villain.
The Waste Lands is great. The character develpment, the tension, the action, the writing, every page is pure entertainment, but...
For the first half of Wizard and Glass I was a little disappointed, but for the last four hundred pages, I couldn't stop and I read it all in a single day. Kind of wish more time was spent with Tick Tock and Flagg though.
Wolves of the Calla is a great mystery/action/fantasy/western, and the fact that it introduces a character from Salem's Lot is great. My only issue with it is that there's a few chapters in between the Priest's tale that feel a bit pointless until the very end.
Song of Susannah is painfully underrated. It may just be build up for the final instalment, but the fact that it is the shortest book since Drawing makes it work a lot better. The tension is great, and you are genuinely worried that not everyone will make it to the end.
The Dark Tower. This book killed me. If you've read it, you'll understand why, espicially the scene where a certain character sacrifices his life to give Roland time to kill another character. Other people are disappointed by the ending, but I thout it was perfect, and like Stephen King says, it's the only ending that really fits.
But I love all these books, and you should binge read them as soon as possible!