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Dynamix; they made the best RPG I've ever played and I replay it every year. This is of course the epic Betrayal at Krondor. They also made the Red Baron games, the excellent and addictive puzzle game called TIM (The Incredible Machine) and the awesome online game Tribes. With almost all their main series, they were immense pioneers and this studio can easily be considered the founding fathers of many modern mechanics and aspects of gaming.
Valve; they need no introduction, in my top three FPS games of all time, Half-Life and its sequel both have a likely permanent position. They've also made the awesome Left 4 Dead series, which is about as much fun as one can have with coop gameplay, in my opinion. Valve too revolutionized an entire genre with their Half-Life games, completely new design as far as pacing, AI and interactivity goes, HL2 almost single handedly introduced physics based gameplay to games with a first person perspective. Besides their developer pedigree, Valve can also easily be attributed with the vast majority of growth in the entire PC gaming market in the past decade or so and Steam has changed the entire global industry in huge ways.
CDProjekt/CDProjekt Red; the Witcher games are something else, literally. They've made three RPG's that manage to defy or naturally avoid most conventiones, especially where quest desgin are concerned. Despite the issues, especially with the first two games, one has to admire their guts for stepping out on a limb and believeing in their product and the massive impact of The Witcher 3 can not be denied, it showed the world that underdogs can step in and show the big boys how it's done. Besides the excellent games, they also started the fantastic service Good Old Games many years ago, and I've since purchased about 80 titles there, they also released a client that works really well. The idea was simple; they started with basically abandonware and then tweaked it for modern OS configurations and sold it cheaply, this has since expanded and CDProjekt along with Valve have been absolutely essential in the big rise of Indie games since the beginning of the 7th gen. To top it off, CDProjekt promotes the best pro-consumer attitude and strategy anyone ever had in the gaming industry and they're just overall a breath of vital and fresh air for gamers all over the world.
Naughty Dog; stuck with them ever since the first PS and Crash Bandicoot. I can hardly imagine a studio which has undergone as dramatic a change as these guys, they went from making games about a furry animal gathering apples and going "ooga booga" to becoming the people behind some of the most intense gaming experiences of our time with massive emphasis on character quality and acting as well as atmosphere. Seeing them pump out new quality IP's every gen is a sight to behold, from Crash through Spyro and the amazing Jak & Daxter and into Uncharted and The Last of Us, they have shown breadth, skill and love for the trade for about two decades now, with no signs of fatigue or drop in quality.
LucasArts; besides the many great Star Wars titles over the years, they were the heroes of most of my friends' childhoods, as well as mine, and the sheer amount of memorable adventure titles in their impressive catalogue is worthy of anyone's respect and awe. Sadly, they have been sold to the most commercial entity in the industry and many licenses are wasted on money-grubbing hogs now, but the plethorea of memories remains.
Bioware; another one that needs to introduction. Their two Baldur's Gate games alone easily net them legendary status in the PC space, and they have several other monster hits as well, including the amazing Neverwinter Nights, Bioware's work on games licensed on the Dungeons & Dragons core rules have made me salivate more than once through the years. This is also a studio that has taken a turn for the condsiderably worse lately, but they have more than enough meat in the freezer to keep me happy overall anyway. Still hoping for the resurrection of the real Bioware, the ones who didn't suffer under the constraints of a looming EA.
Obsidian Entertainment; I can't really think of a mediocre of bad title from this lot. From the amazing KOTOR II, Neverwinter Nights 2 and Fallout: Las Vegas and through to today with epic Indie like titles like Pillars of Eternity, that yanks me back to the days of yore, when RPG's were about terrific writing, characters and depth and not effects and tomfoolery made to impress teenagers with their dad's big screen TV. They are as rock solid as their name would suggest.
Polyphony Digital; yes, they've only ever produced one major franchise, but what a franchise it is. It changed the entire racing genre forever and introduced game mechanics, visual fidelity and depth that no one had dreamed of in the genre before. Even their worst iterations are head & shoulders above most games ever made. The sheer scope of the Gran Turismo IP deserves all the praise in the world, and it becomes evident that the head honcho is a car nut and actual racing driver himself. Been in love with this series since I first rented and later saved up for the first GT, and even with a couple of stumbles, it remains the best racing franchise in the world for me.
Damn, I have several more, maybe I'll add them later, I have a lunch appointment and got a bit carried away... 
