SD is a part of Bethesda, therefore Brink is also a part of Bethesda in some kind of way ^^
Nah, it's 3rd party, just for Brink. SD is actually an independent studio, and I certainly hope they stay that way.
As for my favourite game, I'm currently playing GTA:San Andreas through. Again. I've now bought at least 5 separate times for 3 different platforms, spent hundreds and hundreds of hours just [censored] around, and I've still never reached 100%. Sandbox games FTW. If you take a sandbox and a big chunk of your own imagination, coupled with variety and size in the setting, customisation, and a slew of guns, vehicles, scenarios, and game-altering cheat codes, you get unlimited longevity and thousands of hours of fun.
San Andreas delivers.