Burnham would be a good choice, but I think since he was Chief Secretary to the Treasury in Browns regime, he's a bit too close to the Great Depression and thus probably tainted too much by it. Too easy a target for Cameron.
Same with Cooper, and since her husband is Ed Balls (TM) that's another free goal for Dave.
Umna has withdrawn from the race, which is a shame because he'd probably have been the strongest hope for a Labour that would get votes, clawing it back from the far left into more central ground. Of course, doing that without running into the legacy of Blair and 'New Labour'.
Jeremy Corbyn?
Ain't gonna happen. Far too far left.
Liz Kendall is a potential runner. She would make for a fairly good leader, she's untainted by the 2008 crisis, she's not far left and she's done well in her seat. I think it'll be a two horse race between her and Burnham.
Who would I pick?
Probably Kendall, Burnham is good, but as a member of the financial team on Browns government, that's now become public political suicide. Dave can just bring that up, time and again with his little favourite prop and Labour will not recover from the nose-dive it took this year.
To be honest, I think it's going to take another term before it does fully recover from the mauling its taken anyway, and it'll be at least three terms before the Lib-Dems manage to pull themselves back together, what's left of them.