I had a long time go by when I wasn't involved in the hobby and skipped from 1e to 5e, so I appreciate all the updating of content to 5e from older editions.
That said, all of the publishers you mentioned are also releasing new content as well.
For someone putting out great content that is new for 5e I would keep an eye on Matt Coleville DM (MCDM). The Strongholds & Followers book is great quality. As good as anything Kobold Press, Goodman Games, or Frog God Games is putting out. Even more impressive, given how small MCDM is, he has very active discussion forums for his content (on Reddit) and lots of videos. There are many ways to learn about the content before you buy and to engage with his community after you buy.
Kobold Presses two bestiaries, Tome of Foes and Creature Codex are both excellent as are the lair books that go with them. Many of the monsters are brand new, some created by kickstarter backers. The Lair books provide excellent mini adventures built around the lairs of some of the monsters in the books. They also offer 2D cardboard miniatures for nearly all of the monsters in the books. I've highly enjoyed reading the books and have used many of the monsters as a DM, sometimes just switching out a well-known monster in an adventure with a similar one from the Kobold Press books, to keep the players on their toes, and sometimes I build entire adventures around them--helped with the lair books.
Frog God Games are, yes, printing old material updated to 5e but boy are they offering a LOT of updates. VTT map images, 3D printing tiles, huge cloth maps. Rappan Athuk is impressive, but you really need to look at the awesomeness that is Tegel Manor to see the incredible lengths they are going to bring influential classics to a modern audience.