Based on the date of this review, I would say 1st gen.
Hardware and Game Review: Kinect and ?Kinect Adventures? for XBox 360 | The Flickcast
I know you're going to say that it's not a good idea to be bleeding edge, and you're right. Weigh that, however, against how much I want to have one right now. IwantitIwantitIwantit! <Stomps foot>