Probably 'The Kind Worth Killing' was also quite popular. The unique take on the crime genre, with its morally grey characters and intricate plot, attracted a lot of readers. It was talked about a lot in the crime fiction community in 2015.
'Dark Matter' by Blake Crouch could be another popular one. It combines elements of science fiction with crime fiction. The mind - bending plot and the mystery of what really happened kept readers engaged. It was different from the usual crime fiction, which made it stand out in 2015.
I think 'The Girl on the Train' was extremely popular. It had a huge following and was made into a movie later. Its popularity was due to its relatable characters and the suspenseful plot.