Tempted to say with ease given how Mandrakk II was dispatched by a collection of heroes from the multiverse whereas all of Earth's heroes combined were utterly useless against Nekron.
__________________
"And then there was nothing. A once broken something now void.
And on the first day, Doom spoke... 'Be.'
And then there was life."
Mandrakk II was essentially beaten by Superman wishing for a happy ending from the Miracle Machine (that's how/why Nix Uotan mysteriously popped in and started pulling rabbits out of his hat, ftw.) That being said: a wish from the Miracle Machine would have ended Nekron's rampage as well, me thinks.
Regardless, I'd give the nod to Nekron due to the simple fact that he did more on panel. Yes, Mandrakk II beat Spectre and Radiant, but Nekron also proved that 'Godz powa' is ineffectual against him as well.
Man, Final Crisis had so much shit going on I can't ever remember how and why of everything. I thought the Miracle Machine just undid all the damage? Gotta reread...
__________________
"And then there was nothing. A once broken something now void.
And on the first day, Doom spoke... 'Be.'
And then there was life."