Tough choices, because many of them can be considered for different factors. For example, COD:MW can be considered one of the best come-back installments in a franchise that has since been in decline. But a game like Doom can be considered for revitalizing a style of FPS that has been widely absent since the days of slow-paced FPS game mechanics. And Battlefield 1 was quite revolutionary in the sense that it was one of the first show-pony shooters of its time, thanks to the Frostbite engine.
But overall, I have to give it to Titanfall, probably the most underrated FPS. The game just oozes with the DNA of IW's MW2.