Damn, that's a tough one! From Pandemic, Psygnosis/Studio Liverpool, Neversoft, Looking Glass, Westwood, Ensemble, Midway, Zipper, Troika, and New World Computing, all some of my all time favorite studios, but I would have to go with LucasArts.
From the legendary Adventure games of Monkey Island, Full Throttle, Indiana Jones, Maniac Mansion, and Grim Fandango to the multitude of Star Wars games of the X-Wing series, Rogue Squadron series, Dark Forces series, Galactic Battlegrounds, to even branching out to other genres/new IPs like Outlaws and Gladius, there was SO much that LucasArts had to offer. And they're gaming output was like no other as well.
Although I will pour one out for Bungie, Blizzard, and BioWare. 3 studios that practically are defunct in my eyes.
You called down the thunder, now reap the whirlwind
