Honestly, Bungie. It didn't last but they got Halo, which is why Xbox is still a thing. Wipe Halo from the exclusive list and we're probably looking at a much bleaker history for the Xbox brand. I don't think anything else has that kind of overall effect.
Pretty much this for me. Without Bungie and Halo, I think MS would have scrapped the Xbox project a long time ago.
Naughty Dog easily.
Would have been Bungie but MS lost them. Maybe Bungie became too big for MS to hold onto I guess.
I think it's more of the fact that Bungie wanted to move on from the Halo universe and MS didn't want them to. ND is getting to that point now (i.e. they are a behemoth and their games are selling 10M like TLOU and I believe Uncharted 4 will as well), will Sony be able to hold onto them?