Someone has just asked me this! Such a hard one and obviously everyone is going to have a totally different idea.
I am going with 'Lincoln in the Bardo' by George Saunders, which I know is a 'marmite book' but blew me away when I read it.
I have reread it at least 3 times, gifted and recommended it lots too.
Watching with interest to see other people's choices..