Well, obviously Bethesda and Valve. Those are obvious. I've been fangasming about Skyrim since the day it was announced (or since I finished Oblivion. I didn't know what it would be, but I knew I was going to buy it) and Valve makes enough excellent games that it'd be worth it.
As much as I absolutely hate myself for saying it though, I would buy on for EA. And I would do it because I know, down the road, I will be buying their games. Whether it's a Bioware game or the next Battlefield or the new (and hopefully better, although I supremely doubt it) Command and Conquer games, or even just random games that they end up putting out, it would be worth spending the money up front because it would end up being less expensive than buying all the games individually. I don't think I'd do the same with Activision, although I'd consider it. Depends on what their CoD games end up being like.