Bioware: They just make great games; there's not one Bioware game that I've played and been bored with. Also, the replay value on their games are very high.
id Software comes in a close second. I wish I knew about their games before playing Doom 3, because I bet they would have been way more fun back then. I bought the Doom 3 Collector's Edition and it came with Doom and Doom 2. Those games are great and I still play them from time to time; and also Hexen, Heretic, Quake 1, 2, 3, and 4.
I eagerly await Rage and Doom 4 (new info next year at Quakecon).