I think Chagaev. Ruslan gave Povetkin all he could handle and Povetkin stopped Byrd. Not saying it would be easy, but Byrd has his usual problem in that he doesn't really hit hard enough for heavy and Chagaev will be able to more or less walk through his punches.
Chagaev is mobile enough to get this done. He'd land enough meaningful punches in seven rounds at a guess.
That and DKP did not support Chris to the extent he did his other fighters. Enough to make a 1 or 2 round difference in scoring and that is a big swing with these two guys.
Chagaev. Byrd really isn't on Chagaevs level but his fans very clearly disagree. If you put this to vote Byrd would probably win because hes got a passionate large fanbase.
Byrd. Chagaev wouldn't catch Byrd off guard with his stance, which is how I feel he landed a lot of his biggest shots and Byrd wouldn't stand there and take straight lefts from Chagaev either. I think Byrd would be able to use his defense to make Chagaev look bad and win enough rounds to come out on top. Byrd hadn't had a real good win in a long time by the time he got to Povetkin, anyway. I don't see how that fight can apply as Povetkin was there to be hit, where Byrd wouldn't be.
Byrd easily. Chag left his best in the amateurs was unspectacular in all respects. Byrd too much speed.
The pre 2008 Ruslan, before the blown out Achilles and bout of hep, would edge Byrd. It would be a very interesting fight. Chag was a very skilled guy who knew how to work within in limitations.