11/22/63 was his best, in my opinion. It had a huge scope, probably his best romantic plot, an exciting plot and, what really sets it apart, it didn't fall apart at the end. Some of his other books wrapped up cleanly (The Shining, Revival, Duma Key) but they were smaller scale stories.
After that id say It and The Stand. It just for the scenes with the kids, he could have written that without the monster and it would have been hugely enjoyable. I guess that's what Stand By Me was, but much shorter. The Stand was just a great epic scifi horror story.
Next tier down I'd say The Shining, Duma Key, Revival, Joyland, Under the Dome, Finders Keepers. These are all really good, just lacking a little bit In scope, plot, characters compared with the few I've listed earlier.
A few I didn't like: Mr. Mercedes, Doctor Sleep, Finders Keepers, End of Watch, Bag of Bones.
I also found the Dark Tower series a little tiresome, giving up after book 3. I read them in college though, and was a bit of a snob back then, so I plan to revisit them next time I'm traveling for work.