Galactus. 

What we've seen him do does not put a limitation on what he is able to do.  In his very first appearance, as his ship is landing there is a comment that he "must be a giant", to which the Watcher says something along the lines of "he is whatever he wishes to be - he is Galactus".

If Galactus found it necessary to defeat or destroy the Celestials, then I believe he would do so. 

Good thoughts Kip. I had totally forgotten that Korvac had orignally gotten his original powers from Galactus' own ship. And that's what I was not sure about either...as Korvac defeated more and more people he seemed to be gaining more and more strength.

There's also times when Galactus is very weak too. During those times I think the Celestials could possibly win I guess.

Like for example in JB's run in FF when Thor had to use the powers of his Uru hammer to power up Galactus again. That was too cool when Galactus started to unknowingly siphon the power of Thor's Uru hammer via Mr. Fantastic's machinery...so much so that Thor himself couldn't pull it away! And this was after Galactus had already siphoned power from Terex IIRC. I'm not sure how Galactus would do against the Celestials then. It seems to me that Galactus would lose for sure.*

I think it's ultimately up the whims of the writers at the time who would win...and how consistent they are with their Pre-history. It's inevitable with 40 years of history that some inconsistencies will happen. But man, those are the stories that had some real kick to them!

*PS. What exactly would happen if Galactus died? I remember a few years ago [gulp...that was 10 years ago...sheesh] there was a run with Galactus skull crashing onto Earth or something...

 ...But the universe never imploded in a BIG CRUNCH because of it. Again, I think it depends on the whims of the writers at the time. But I was really surpised about that story (and I wasn't even colecting comics anymore then!).

Or there's all that bizarro stuff in the EARTH-X et al. series written by Jim Krueger in which Galactus's job has always really been to

SPOILER SPACE

kill the Celestial eggs that are hidden in the cores of various planets. And Franklin Richards BECOMES Galactus. And the Watcher is really an evil pawn of the evil Celestials.

Whee!

Cheers, Jon

Wow Jonathan, I must be totally out of the loop. That is totally bizarro world stuff to me. I wasn't aware of any of those story lines. 

Bingo Kip, that's why I was bringing up before with the weakened Galactus. It's hard to say how it would play out and I think it depends on the writer.

At one time, when weakened, Galactus can be knocked unconscious with the Thing being elasto-banded into his face by Mr. Fantastic. At another time, when empowered, Galactus can stagnate the flow of a huge black hole and allow people inside to escape to freedom.

The variations in his power level are very extreme from time to time.

QUOTE: What exactly would happen if Galactus died?

Some things are constant in this Universe. Me answering the question above with the following, is one of those constants.

Nice collection piece William! That is some nice JB work there!

Edit: Dang! That is very INSPIRING WORK! Great stuff!