Killzone. I really enjoyed the games on PS2 and PSP, but starting with PS3 the series took a downward turn and never recovered (don't kill me Miguel!). Mercenary on Vita was good, but Cambridge's reward was closure :(
Halo. This one breaks my heart. The first Halo game is in my top 5 games. I consider it the best FPS franchise. Yet once Bungie walked away it was never the same. 343 took the series in a lot of strange, counter-productive directions.
Before 2016 I might have said Call of Duty or Resident Evil, but Infinite Warfare and Residnet Evil 7 renewed my faithfulness. As for Gears, I actually think it's in really good hands, minus the microtransaction BS. Fergusson did a great job with 4.