Gonna get some strange faces with all the Molyneux hate arround but for me it was Lionhead.
They were being really open to their community, and a couple of their devs would come into #Lionhead on Quakenet and we'd talk about stuff they were doing etc. They also had webcams set arround the office online 24/7. Gave you a real look into the studio, made everything more personal, it was great.
Not much left of that though: the webcams stopped refreshing quite a long time ago and while there are still a bunch of people on the IRC channel, not much devtalk going on anymore. I blame Microsoft. ;(
The smaller studios tend to actually communicate towards the fans, make you feel welcome and your opinion, if constructive, valued.