id say galactus at normal can take on one or two celestials, maybe 3. he spends his life eating their babies, if they could do something about it im sure they would. at full power i think he could take on the majority of them at once.
hes supposed to be up there with death and eternity but they always have him doing the job to some cosmic being. in deadpool there was a weird space thing that was compaired by a watcher as being stronger than even galactus. his abilities are written poorly in general and never given the respect he deserves.