BCA Galactus had a lot of potential, but in the end his best feat(key word) was absorbing Tiamut:
http://i.imgur.com/qMFW44E.jpg
http://i.imgur.com/t5CTYV8.jpg
However, Tiamut was nearly powerless at the time, which makes the feat MUCH less impressive:
http://i.imgur.com/KDyN1Ef.jpg
On the other hand, Mageddon destroyed Wonderworld and all of its Godlike, hyper-dimensional inhabitants(aside from Glimmer, who only escaped by running perpendicular to time.) And in DC(especially under Morrison), higher-dimensional status typically puts you leaps and bounds beyond anything from the lower dimensions. For example, the beings of Wonderworld were so advanced/powerful that they contained an infant universe in a petri dish, and were able to imprison a 5D Imp inside a 6D bottle -- this was all on display in their museum, lol:
http://i.imgur.com/wkrpSjD.jpg
Heck, even Glimmer(Wonderworld's sole survivor) possessed enough energy on his own to easily convert the entire earth's populace into Superman-level beings...Yet even he was forced to flee from Mageddon(obviously):
http://i.imgur.com/XGkKvah.jpg
http://i.imgur.com/md5YsZv.jpg
http://i.imgur.com/bu5GLxO.jpg
*And remember: Mageddon wiped out nearly the entire race of 'Wonderworldians' solo.
Additionally, the Pax Dei(the literal army of God) were so certain that Mageddon was going to destroy the universe that they completely abandoned it, and had actually began engineering an entirely new universe to replace it:
http://i.imgur.com/uIRzhw0.jpg
http://i.imgur.com/9NoU6af.jpg