I will say that both The Mist and The Shining are the best adaptations, but when it comes to which one is the best, I have to say it's a close tie.
On the one hand, The Shining is a great example of how the adaptation can sometimes exceed the original source material, Stanley Kubrick is brilliant when it comes to pacing and composition, and it was genuinely creepy(I did not see the meaning of "redrum" coming, Jack Nicholson is fantastic in this, etc.). Not to mention that it features the one moment in any film I've ever seen where I was both aroused but terrified at the same time (you know it if you've seen it). Not to mention that the ending just blows your mind (I still don't know what it means).
On the other hand, The Mist has some of the best monsters in any film of the last decade, plenty of great acting, and the very notion that biggest terror in the film isn't so much the monsters, but some of the people in this movie. Basically, it's a great example of a potential scenario in which humanity finds itself threatened yet we're all still the same selfish, arrogant, high-minded jerks people normally are. Also, the ending is a huge slap to the face. Seriously, it will kick your ass and piss you off. In a good way.