Mine was also The Witcher 3: Wild Hunt. Awesome gameplay, epic story and probably one of the best looking games ever (on PC of course).
Some honourable mentions for me:
Metal Gear Solid V: The Phamtom Pain
Batman: Arkham Knight
P.S The Witcher 2 is pretty good but not as open world as The Witcher 3..and the first Witcher is pretty bad...
Another one that I really liked was Factorio. A super indie game - but if you're into base building type games at all, Factorio is wonderful😃
It's one of those early access games that is still in alph and has been for ages. But I was plenty happy with it at it's level of completeness a few months ago.