I have to agree with BBD. You watch Matt Ryan and the anticipation and quickness and aptness of his reads immediately stands out at you. It's the same thing that always made Peyton Manning my favorite quarterback to watch. Ryan is already playing like you hope the young quarterback on your team will play when the "light comes on" or "he puts it all together". Ryan seemed to hit that point immediately. I really like him coming into this past draft, but there's no way I could have predicted he'd be this good this quick.
Flacco is, at times, playing really well. But there are times he looks lost and his confidence crumbles a bit. At his best, he looks like a great deep passer with strong athleticism and decent decision making. At his worst, he looks a bit like a guy who's in a little over his head. Those bad times will pass, to be sure. Baltimore seems to have found their franchise passer (and if they get a good deep threat on that team, watch out.). But, in watching them, there's no doubt in my mind that Ryan is the more impressive right now.
BBD is right. He's the most impressive rookie quarterback I've ever seen.