Square Enix - They've made some of my favourite games of all time: FFVI, IX, Chrono Trigger, as well as other stellar titles like Parasite Eve, and Einhander.
Konami - MGS is one of my favourite series of all time, and they make the only survival horror series worth playing, Silent Hill. Not to mention Castlevania SOTN, and the older Contra games.
Namco - They make it on here for Soul Calibur alone. Surprisingly, I can't even think of another Namco game or series that I even like.
And finally, Sony - I'm going to lump all their first and second party developers together since I can't be bothered to list them all. Sony gave me Gran Turismo, God of War, Ico, Shadow of the Colossus, and the Jak series. In other words, they were making the first party titles I enjoyed the most this past gen (and in the case of Ico and Shadow, two of the best games I've ever played). Sony doesn't get nearly enough credit for the stellar titles it's developers have put out.