I recently finished the book series and I think I might have a fairly controversial ranking of them all because I thought the last two books went HARD. I get the meta aspect bugged some people (and yet it's built into the fabric of the story from the very start so I don't really get that) and maybe if I had been waiting for years in anticipation of the conclusion I would have been let down or something (I read them all in the past year or so) but, yeah, the beginning and the ending is where this thing really sung for me. The fatalistic/KA-talistic vibe of the final book and the mood of that whole thing plus its setpieces and ideas... gawd-bomb finale. Ka is a wheel, do ya ken it.
1. Song of Susannah
2. The Dark Tower
3. The Gunslinger
4. The Drawing of the Three
5. The Waste Lands
6. Wolves of the Calla
7. Wizard and Glass
That's not to say I thought Wizard and Glass was bad, I'm just not sure the flashback back-story needed to be 600 pages, say thankya. Looking back at it all in retrospect I think it is VERY smart they are starting the TV series with the back-story, though.