What mechanics are the most fun, innovative or immersive?
I'll start with my list.
Enemies reacting to your gameplay choices in MGS V
I still think that game is a masterpiece when it comes to stealth gameplay and generally reaxtive gameplay but I thought that the system of enemies adapting to your gameplay choices was particularly clever.
The parkour system in Dying Light
I don't think any other game has pulled this off that well. Combined with the stressful night sequences this made that game a sleeper hit for me.
The lack of weapons in Subnautica
This is what turned this into one of my favourites of this year. It really makes you feel vulnerable, especially in the beginning, making for some very atmospheric and creepy gameplay.