I contend that Molecule Man is THE most powerful being in the Marvel Universe (except for the Beyonder). I am in the process of going through my old comics and I was re-reading some Secret Wars. In secret wars II #6, Uatu says (talking about molecule man) "he is mightier than all other beings in the multiverse". While he says this the background shows death, mephisto, in-betweener, eternity, one-above-all(celestial) and even Living Tribunal. Uatu also tells Molecule man (talking about the beyonder) "You alone might stop him." Later in the same comic Kronos, Death, Chaos, Order, Eternity, In betweener, one above all, Gardener, Death and The Living Tribunal all appear and beg beyonder not to kill death. Beyonder ignores them and does it anyway (no one even tries to stop him because they know they can't). So Beyonder is the most powerful being in the Marvel Universe followed by Molecule Man.