Losing Square Enix would suck the most.

They are a publisher.

Their individual IPs are headed by completely different dev studios. 

I guess Bethesda. As much as they have been an absolute joke for the past year, I'd miss Doom and the Elder Scrolls series. Plus, they've probably given more support to the Switch than any other major third party publisher.

Platinum would really be a shame to lose.

^ I dont agree with this.

Gameing would be worse off without a Sony or Nintendo.

Would majorly suck.

People keep posting publishing studios of individual Dev teams lol.

Hmm. I'll go with whichever development studio handles the main Zelda games. A world with no groundbreaking Zelda games, generation after generation, sounds sad gaming wise.

Paradox Interactive and Devolver Digital come to my mind. The former would even extend to tabletop gaming as they own White Wolf Publishing, creators of the World of Darkness series of games.

Name one studio that fufills the criteria of churning out several games per year. That was feasible 25 years ago, but nowadays, not so much anymore. If it brings out several games in a year, it's a publisher, not just a development studio, because even studios with several hundreds of developing staff can't produce fast enough to do this.

Sega.

I'm just looking at my Switch third party titles and I have both Puyo Puyo Tetris and Valkyria Chronicles 4, both of which are awesome games.  I will likely get Sonic Mania at some point too.  Sega may not be first party anymore but they still have some of that original first party mojo left in them.

(For all the people looking for "developers" instead of pushlishers, then consider my answer "Sonic Team".)

I don't know about industry as a whole.
But for me personally, it may be Capcom or Square Enix.

Sega/Atlus