Black Celestial arc Galactus eats the marvel omniverse..
This is basically the same Galactus that was shown in BCA who was eating all reality. Let's say he succeeds and as well as the omniverse eats Tiamut and then moves on to DC universe. Now who on my list can defeat a Galactus who has eaten the marvel omniverse from my list.
Please note that Galactus actually fights back and is not a brainless zombie. THIS IS NOT A GAUNTLET and Galactus's oponents are NOT listed in order of power
1-Kismet
2-Dominus
3-Hourman
4-Amazo
5-Pre-crisis Darkseid
6-Yuga Kahn
7-Spectre without backing of the presense
8-Shazam(fight takes place near/in rock of eternity)
9-Destiny
10-Spectre with backing
11-CoIE Anti-monitor
12-Mxy(not WF)
13-The Ultimator
14-Mandrakk
15-Cosmic Armor Superman
16-Original Parallax
17-Original Ion
Amazo: Kind of flipping a coin, one would eat the other.
Spectre not backed: Not sure if Galactus can eat the Spectre because of his nature, if we treat it as a regular abstract Galan wins.
Spectre with backing: Realistically wins with ease.
CoIE Anti-monitor: They would be pretty much around the same powerlevels, I think the Anti-monitor is more flexible and would eventually overpower Galactus simply because he is mad.
The Ultimator: I'm going to assume she can destroy Galactus before he becomes a threat although she could be destroyed herself.
Mandrakk: Again, he does the same thing more or less, flip a coin or consider this like the Anti-monitor.
Cosmic Armor Superman: Already defeated pretty much the same opponent.
Amazo has copied the powers of the Worlogg, so why not copy Galactus's powers and do the same? If he lacks of knowledge (Galan is implied to be omniscient of sorts) and it plays an important part in the battle (which may not be the case given that Galactus was crazy) then he loses. Otherwise its pretty much the same battle.
Mandrakk "eating stories" is pretty much the same thing than eating the Universe which is what this Galactus does. You can think it as something bigger (eating a shadow form that serves as a foundation of the universe) or something less (eating the archetypical form of the universe and stripping it from its form) but they are both speculations. We can say that eating everything the story is, means eating the story, so Mandrakk and Galactus pretty much mimic each other.