My teacher mentioned this and I'll say here what I said to her here: polls don't matter now, they'll change.
Walker has the best chance to beat Doyle because Doyle needs heavy returns in this area to win the state. (Milwaukee area and suburbs so Southeastern Wisconsin) Walker won re-election as Milwaukee County Executive IIRC with 53%, enough to turn this area and the election to him if he wins the Republican nomination.
But would Walker have a harder time reaching out to rural voters in the North Country than Green? Would Doyle be able to pick up healthy margins in Western counties and maybe even win in Green's base?