Because I don't think in one-best terms. Different books can be great to me for different reasons.
Do you mean best in literary quality or best as in what engaged my interest the most? Does it count if competing books engage me equally, but for different reasons?
If I absolutely had to settle on one, then for now, it would be The Violin Conspiracy by Brendan Slocumb, for its deft weaving of classical music, social justice issues, and a good old fashioned mystery. The writing is quite good--maybe not literary quality, but not far off, either. Of course the characters and sense of place are also terrific.
But I could just as easily have said that it was No Game for a Dame by M Ruth Myers, because the Depression-era heroine and how she navigated her mystery were so engaging and fun.
Or I could have said Convenience Store Woman by Murata Sakaya. It's a "literary" novel that wasn't a pain to get through, about an Asperger's woman in Tokyo who loves her job. Problem: Those around her, and even the very culture itself, tries to tell her she shouldn't like that job that fulfills her, that she should have different goals, "better" goals. Her journey of self-discovery was a thoughtful read, yet enjoyable, too.
Or I could have named any of the Brother Cadfael mysteries I've read this year, because I enjoyed the puzzles of the mysteries, the ingeniousness of the crimes committed, the immersion in the historical period when it takes place, and of course Brother Cadfael himself.
So any of those would work for me.