Id, Bioware, and Bethesda have all been mentioned, and are strong contenders.

Ludeon Studios and Ghost Ship Games each only really have one game (RimWorld and Deep Rock Galactic, respectively), but they’re so damn good and they both have such a strong relationship with their communities that they’re definitely among my favorites at the moment.

I have a love/hate relationship with Paradox Interactive. Their DLC model… leaves a lot to be desired, but I have yet to find much else that scratches the grand strategy itch in the same way they do, and the level of mod support they provide is impressive. I don’t know if I can call them my favorite, but I’ve logged over 1000 hours into EU4 alone, so I have to at least mention it, I feel.