Missouri - Nixon is too damaged now to beat Blunt, but Clint Zweifel and Jason Kander would be the top 2 picks. One could run against Blunt and one could run for the open Governor seat.
Chris Koster, the AG, seems to be most interested in the Governor's seat. I don't think Nixon is necessarily too damaged now, as the election is two years away and this (unfortunately) won't be a salient issue in 2016 that many voters are thinking about. But if Nixon doesn't run, Kander would probably be best for the Senate seat and Zweifel could run for another statewide office, maybe LG.