Interplay, Microprose, Lucasarts, Valve, 2K, THQ and lately Sega surprisingly. Oh and Origin before they got eaten by EA.
Some don't exist anymore but they deserve to be mentioned.
Interplay drove innovation forward, mostly known for RPG releases.
Microprose always represented solid design, and is of course best known as the company Sid Meier was a partner of.
Lucasarts for releasing some of the best point and click adventures and always striving to be better.
Valve for reaching out towards its consumers and fans.
2K, THQ and Sega for their variety of products and usually solid quality, and they rarely outright make me rage.