I'd go with Capcom, since they've provided me with some good survival horror titles over the years.

Capcom? Also Respawn.

My favorite dev is gone (Visceral Games) with no noteworthy dev taking their place.

Sega

Respawn, some of the Ubisoft studios, come to mind. For me personally, the biggest loss would be Bungie, as I am a Destiny fan.

CDPR would be significant as well, considering that they're responsible for one of the best games of this gen, and have probably the most hotly anticipated game slated to come out next year. The Witcher is now a hot property, and expectations for the next TW game will be huge. It would suck if that game, or Cyperpunk (after all this waiting) didn't happen.

I'm pretty sure most of their flagship bi-annual IP are actually made in collaboration with their studios working together. Either way, they own all of the studios that work on their games, and I think most if not all of them even have the Ubisoft name. It seems like they run their studios a lot more like say Nintendo EPD is run, rather than say Retro or Monolith to Nintendo, only with entire studios instead of teams within a larger studio. 

 Also they own the studios so they're really a developer and publisher. But if you took the OP to only mean single studio developers, or developers who can't self-publish, I understand. 

Sega as a whole.

We'd lose Total War, Yakuza, Persona/SMT, Valkyria Chronicles, Etrian Odyssey, and Two Points Hospital.

I vote for Capcom.

Happily for us gamers, there aren't that many eggs in a single basket. We'd be fine without Monster Hunter, Resident Evil, Street Fighter and the like. But it would be the one I personally think would be the most felt given the loss.

As far as I'm concerned, we have already lost them.

SE hasn't made a single thing worth a dime from me since they released Final Fantasy XII.

Dragon Quest XI was amazing, but everything FF since XII has been crap.