David Miliband and Ed Balls cannot stand as they are not a current Labour MP, and nobody available is as good as Ed Miliband. That may sound strange to some, but the was never actually anything wrong with Ed and his resignation was a big mistake, he didn't fail his party, his party failed him.
The right wing media made him out to be a loser, a fool, halfwit, weirdo, etc and his party failed miserably to dispel that myth. Even in to the last year senior Labour officials were maintaining a view that the public would warm to him and they just needed to show the people the "real Ed" and they were 100% correct, the problem however was they never did it, the public never met the real Ed until he sat down with Russell Brand a week before the election in an interview nobody watched.
If they had let people see the real Ed, a competent/confident politician and probably the nicest most genuine person to ever run for Prime Minister then they would absolutely have warmed to him. The problem was they tried to fight a 20th century campaign with a 21t century leader. If they had stuck him on TV shows and had him interviewed on late night shows they could have got him in the public eye and people would have said "hang on, this guys not an idiot, and he's much better than Cameron", it could have been another 1997 style landslide, however it turned into the worst defeat in decades and the person least responsible took all the blame.
Unless Labour learn from their mistake and stop blaming Ed for their faults they will not be able to progress.