Please note: the order is thematic rather than ranked. Several of these are rereads, and many of them were published prior to 2015.
We Were Liars by E. Lockhart
The Library at Mount Char by Scott Hawkins
Feed by Mira Grant
The Technician by Neal Asher
F/SF (note that all of the above could also plausibly go into this category)
Armada by Ernest Cline
The Shepherd's Crown by Terry Pratchett
The Bees by Laline Paull
The Goblin Emperor by Katherine Addison
Ancillary Mercy by Ann Leckie
Uprooted by Naomi Novik
The Year We Fell Down by Sarina Bowen
The Chocolate Temptation by Laura Florand
Two of a Kind by Susan Mallery
The Clock Strikes Twelve by Patricia Wentworth
Ghosts of Belfast by Stuart Neville
My summary statistics for the year: 147 books read: 116 new and 31 rereads. 41 new authors tried: from above, that includes Neville, Mallery, Florand, Bowen, Cline, Hawkins, Paull, Addison, and Lockhart.
I'm putting together on a separate post on how my Hugo-nomination list is shaping up.