There's this rule about GAF first replies always being correct.
Some of my favourite games of the year were exclusive to it:
Pillars of Eternity
Shadowrun: Hong Kong
The Age of Decadence
Cities: Skylines
Contradiction
Guild Wars 2: Heart of Thorns
(Note that I won't attempt a full listing of all good PC exclusives released in 2015, that would be silly
It also had some of the very best "exclusives" of other platforms, like the excellent Ori and the Blind Forest and Rocket League. And of course, the best version of the vast majority of multiplatform games.