I have always been a fan of the Celestials. So my answer is simple. Arishem the Judge. The Celestials are a star faring face. Have a bond to Eternity its been said. Galactus is the third force that bridges Death and Eternity. He views them as siblings. Same could be said the bond with Eternity and the Celestials. It would shake the universe, destroy so many but i believe Arishem at full strength vs a full-powered Galactus(this is in theory, we have never seen a Celestial at full strength and will never see a full powered Galactus) will win.
Yes on everything you said about Galactus. Which is why I think he will win. But I have not heard it said explicitly that the Celestials consider Death and Eternity as siblings (and vice versa).
The Celestials are on the top level of non-abstract beings in this universe. Galactus is on the level of the abstract beings, though arguably a bit lower (on the basis of how many comic writers use him as a test to prove the power of their characters).
Galactus is above beings like the Stranger. While Galactus has been defeated by earthly heroes at times, so have the Celestials (infamous Invisible Woman incident). Think Galactus can't make a force field like the Invisible Woman, but on a much grander scale? Why couldn't he, with the power cosmic, just duplicate whatever cosmic rays gave her her power. If he can make the Silver Surfer he can do that.
In response to seaapple, how do u think Galactus fares against the Celestials. As powerful as Galactus is, even he would fall before the combined might of the Celestials. I like Arishem winning over Big G