Personally the first hour of the movie when it was super slow was my favorite part. I fucking LOVED every second of the first hour. As soon as act 3 started and people started actually being possessed and such I stopped caring as much.
The ambiance was great and the acting was as well.
Then again... All my favorite movies are basically just a small group of people talking in a room for 90 minutes so it makes sense that my favorite parts were when a small group of people talked in a room for 90 minutes.