I think klei may be my favourite studio. They create amazing games that are all truly unique but backed by strong game design concepts.
Mark of the ninja - great 2d stealth game were most games in the genre are 3d.
Invisible Inc - great turn-based Rougelike stealth game.
Don’t starve - probably one of the best survival games out there that relies on clever resource management rather than combat
Oxygen not included - a base building survival game, that is well designed to ramp up difficulty with the long term needs of your base
Griftlands - deck building game with a charming plot and interesting mechanics.
Each game they output is truly unique and interesting experience with some really clever design choices, but I think the point they became my favourite studio is when I read their article on Intrinsic vs. Extrinsic rewards to appreciate how well thought out their games were.