Galactus, by far.
Galactus is out of Dr. Manhattan's league. idk if he'd even make it past the heralds.
I see him closer to Silver Surfer's power level, Galactus would be in a different league indeed.
Manhattan was "created" in a lab accident, Galactus was created with the destruction of his whole universe, the energy difference between both accidents is just too big.
Manhattan can create a big sand castle on Mars, while Galactus can just devour Mars, or Jupiter, or Saturn, or even bigger planets outside our solar system. He could just eat Earth with Manhattan on it, and the doctor's only choice to survive would be to teleport outside the planet, he couldn't fight against one of the most powerful beings in the whole universe.
If I remember well, Mahattan himself said that his powers had certain limitations, that he wouldn't be able to stop a soviet full nuclear attack, I don't remeber if he said exactly this, but I remember reading (like yesterday) something like this mentioned in a thread from other forum where a similar fight was discussed.
Galactus not only has the power to stop that kind of attack, he has the power to destroy and devour full planets, he is among the most powerful entities in the whole universe, Manhattan could be just a powerful herald to serve the devourer of worlds, not a real threat, at least not when he is full powered and not starving.