The last novel of King's I read was 11/22/63, I thought it was pretty awesome. I guess I should have recommended the classics first. They've become such an integral part of my literary history that I don't even think of them as books that it's possible someone hasn't read, even when implicitly stated. I've been reading King since I was about 12 years old, which I'm sure has ****ed me up beyond repair, and I can't believe my parents allowed me to read him at that age, although I'm grateful that they did.
King's 'newer' novels take a much different track than his earlier stuff. I would call the turning point 'Gerald's Game' (which was awful in my opinion and the opinion of many others, bad story and also just plain stomach-turning). Anyway - 'IT' for sure, 'The Shining', 'The Stand', the 'Night Shift' collection of short stories, 'The Talisman' with Peter Straub (that one's brilliant!), I think "Tommy-Knockers" wasn't his best book but very engaging, I'm sure I'm missing a few here. In addition to 'Insomnia', reading the 'Richard Bachman' novel "Desperation" in conjunction with the King novel "The Regulators" can be very rewarding, he did some interesting things with plot and character relation between the two books. "The Long Walk" is a fantastic short story, makes "Hunger Games" look unoriginal and silly. Although his earlier novels such as 'Carrie' and 'Salem's Lot' appeal less to me as they are more categorically 'horror' than his later novels which explore much more about the human psyche, the mysteries of space and time, and stretch his place within his subscribed genre - they are still classics and really enjoyable reads.
Another short story collection he wrote, 'Full Dark, No Stars" is an exceptional piece of work, after reading you might be in awe of how he brings the first story around full circle to the last while incorporating the stories in between, all of which are seemingly unrelated unless read together in order.
I also think 'Duma Key' was a little under-rated, I really enjoyed that one a few years ago too. There are also several you may want to read for cultural reference, such as 'Misery'. I could honestly go on and on about my reflections on King's work but that's probably a topic for another forum. I just highly recommend reading him if you're someone that enjoys literature that is (for the most part) easy to digest and at the same time intellectually stimulating.
I bet 'Dreamcatcher' is full of typos, I haven't dared to read it since the first draft. I'm afraid I will be severely disappointed in my proof-reading skills.