I share the sentiment of another poster, i.e. that since it is one long story, it is difficult for me to break it down for ranking. However, in the spirit of the exercise I will attempt to do so. Normally, when I start one I have to reread all of them. My ranking below is based entirely on impulse reads. In short, of the set which are the ones I'm most likely to just grab for a quick reread on a whim. I assume this is the closest I can come to playing favorites.
1. It is a tie between The Gunslinger and Drawing of the Three.
*The reason I can't really weight one of these above the other is because they are so different in tone. It would be like trying to compare the movies Alien and Aliens or The Terminator and its sequel. The first in both cases is a horror movie while the second an action adventure. When I feel the need for a fast and dirty Dark Tower fix, it is usually one of these two I start to read. In a way, they are both the "beginning" of the story. The Gunslinger comes across to me as literary fiction that also has one hell of a good story installed. Who says you can't have both? Drawing of the Three changes tone and perhaps takes the language down a notch. I think this is fitting because it encompasses more characters. We have moved beyond the limited, alien mindset that the Gunslinger has adopted due to his strange life. When the story revolved entirely around Roland, the language and feel needed to feel as though it where the "High Speech." Once it came to involve his growing Ka-tet, the language had to step down because it was about a group sharing the story and eventually their Khef.
2. Wizard and Glass and The Wind Through the Keyhole
*I probably lump these two together because they both involve young Roland and company. They are easy to pick up and read without going through the entire set because they are stories within stories, and seem to stand alone in that sense.
3. The Wastelands
4. Song of Suzanna
5. The Dark Tower
6. Wolves of the Calla
*The final listing still feels wrong and kind of arbitrary to me. To me it seems totally impossible to separate Song of Suzanna and The Dark Tower. While I love Wolves of the Calla, I've never once picked it up just to read out of order. It has always been part of a reading binge.