"I am Legend" by Richard Matheson is probably my favorite fictional book.
Otherwise I read a lot of patristics like Ss. Basil the Great, John Chrysostom, and Isaac the Syrian. The latter is probably my favorite because of the way he combined deep theology with monastic ideals.
I haven't read a book in whole or book series for a few months due to having to read so many super dry journal articles/textbooks for school assignments but the last series I remember reading and really enjoying was the Delirium series by Lauren Oliver, so Delirium, Panemonium, and requiem. All of which were great reads. :)
The Harry Potter series is always gonna be my #1 since it was a part of part of my childhood, adolescence, and adulthood and I shared the phenomenon with my sisters, cousins, friends, co-workers and strangers of all ages.
Other series of my childhood that will always be dear to me are The Bailey School Kids (my first series) and Goosebumps.