The most common answer is Russ Feingold. But as this piece points out, he has given no indication he's interested. There is no denying Feingold is the strongest possible candidate for Dems. And I would never say at this early date that he couldn't be Scott Walker. The 250,000 jobs goal is going to be a self-imposed Sword of Damocles libs will be more than willing to hang over his head.
But it's likely the national economy will continue to be anemic through 2014 and if Walker's reforms give signs that Wisconsin is headed in the right direction and if the state outperforms the national economy, that may be enough. But know this; while Walker's position is strengthened because of his recall victory, so is the venomous hatred of him from the left. I think whether they have a credible chance hinges on a Feingold run. Ron Kind's name keeps coming up for statewide races, but he seems content to stay in the House of Representatives.
Based on what I've been told by those who know Feingold(ranging from somewhat to very well), it's unlikely he'd challenge Walker in 2014. I think it's highly unlikely, but I suppose you should never say never. But there's no question Democrats made Walker even stronger than he would have been by pushing the recall. And Tom Barrett showed how ineffective an "anybody but Scott Walker" campaign can be.