blizz for diablo n SC, id say wc3 to but i mainly like the custom maps witch art rly their work. they did however make all that posseble, but yha still blizz.
Capcom. They release a variety of of games from different genre types and almost all are good games. They constantly release good sequels to their successive titles while making sure to stay fresh and make new game series that end up being pretty popular as well. Seriously, how many companies consistently release a good amount of games to keep us occupied? Also how many of those companies don't just milk the sh*t out of one of their successful titles only? Capcom makes sure to bring a variety to their customers.
Sony
Nintendo
Rockstar
Square-soft(RIP)
Midway(RIP)
Sony and Nintendo for all the excellent and unique exclusives they have. Rockstar for GTA & Midnight club. Square soft for FF series and Midway for mortal kombat. I have others but these are my tops.
Blizzard for SC and Diablo..... Rockstar for all the gta series and some other awesome games like red dead ... and namco for tales and some other good ones :)
Valve, Betheseda is great too, and of course Ion storm for Deus EX. And of course I can't forget about Banpresto for SRT, and Atlus is a publisher not a dev, but great nonetheless for bringing over so many jrpgs.