I pretend nothing! I am SUPREME!! In my native realm I WAS the universe! -- Beyonder
Beyonder is supose to represent the power the writers themselves have over the comics. ~ Stan Lee
The Beyonder was an extra-dimensional entity which was the sum of everything in the universe known as the Beyond-realm, and became aware of our universe/multiverse due to Owen's accident. The Beyond-realm (which is the sum of everything the Beyonder is) is a universe/multiverse beyond time and space, to which our own is but a droplet of water in an ocean when compared to it.
Classic Beyonder is the strongest being in the Marvel Universe. He is Millions of Times More Powerful than ALL the Rest of the infinite Multiverse Combined. Just by thinking he could destroy the entire Marvel Multiverse. In the secret wars series he gives the Multiverse 24 hours of time before he will erase it. Molecule Man blasted Beyonder with a shot which could have, “Slagged several Billion ENTIRE Dimensions (Universes)"
All this blast did to Beyonder, was make him cough a little bit.
He was so strong, that all the Marvel abstracts (such as Eternity, Death, Infinity,etc..), supreme comsmic beings (such as Galactus, Stranger), and The Living Tribunal himself were left at his mercy.
Beyonder is too big for the multiverse, his very presence alters the entire multiverse a great deal, without him even knowing it.
Beyonder puts a large portion of his power in the cup and forces Death to drink it, erasing the concept of death across all the multiverse. Then he brings her back with Dave's sacrifice.
Even with a small fraction of his power, Beyonder is still more powerful than the entire cosmic assembly (which includes the Living tribunal and Eternity). He did destroys parallel dimensions/universes just by standing there. He proceeds to erase a thousand galactic races (including their home worlds) out of existence with a mere shrug. In his universe, he is the one and the many, the nothing and the all, he had no beginning nor will he have an end.
According to Stan Lee, Beyonder was TOAA of his own actuality (like TOAA in his actuality Marvel, and Presence in his actuality DC, and Man Of Miracles in his actuality Image), the purpose of his original existence was to explain the existence of the other comic companies and their own universal laws and structure with their own supreme beings.
According to Jim Shooter:
The Marvel Universe was like Paramecium (a single cell organism)
swimming in a drop of water, while Beyonder was an infinite Multiverse. He could beat the ENTIRE Marvel Universe (except TOAA,Fulcrum)at once.
This version is considered to be Omnipotent. Writers later on depowered The Beyonder, which resulted with other versions. Marvel had two God's, then changed to One Above All as one God. Classic Beyonder was equal to TOAA.