What, only one?
At the top of my list based on quality, interaction with players, and outstanding level and volume of support, it has to be Paizo.
They have multiple monthly releases supporting the Pathfinder RPG, most of which are decent to amazing. Rather than resting comfortably on their proven system they put out some kind of experimental type mechanics or adventure every year - examples include Unchained, Iron gods, and Mythic Adventures - that give players an option to change things up. They also support it with miniatures, map packs, cards, and a ton of interaction with the people who are playing their games and using this stuff, allowing us to shape the direction of the game as it goes along in a very real way. It's a big job and they've managed to stay on top of it better and for longer than any company I can recall.
First Runner Up: Pinnacle
They have a great system in Savage Worlds, interesting settings in Deadlands, Neccessary Evil, Hell on Earth, 50 Fathoms, etc. and they've been keeping them in print for 20 years now. They've been using Kickstarters quite a bit the last few years for big projects and they have delivered on every one. They are active on forums and mailing lists and do seem interested in what players want. The bottom line is that I like almost everything they release and that's rare.
Second Runner Up: Green Ronin
They publish M&M and have done so for years. They published Freeport as well and I have spent some time running games in that setting too. Their releases are high quality and high utility as well in my experience. The only knock on them is that the last few years have expanded their focus to some games I do not play which has delayed some releases for the games I do. Beyond that, I like a whole lot of what they do and the way they do it.
Dark Horse Entry: Fainting Goat Games
These guys publish a whole line of superhero adventures and setting material. It's mainly for ICONs but they have branched out into other games as well. They put out interesting material, typically at a bargain price, and their whole approach just screams fun. Their DTRPG page pretty much covers it
These guys also run/show up on the BAMF podcast which is a favorite of mine and you can find it over there on the right.