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Do you mean Pre-Retcon, or Post-Retcon Beyonder?
If you mean, Pre-Retcon Beyonder , then click on the link and you will see that this is a horrible, horrible curb-stomp (which in itself is a huge understatement) of omniversal proportions. Pre-Retcon (or "PR" Beyonder, also called Classic Beyonder and Secret Wars Beyonder) is widely considered to be (besides possibly TOAA or Thanos with HOTI, both issues which are still being very heavily debated) the most powerful character Marvel's ever created and is highly ranked amongst the most power comic book (if not all around fiction) characters ever created
If you mean Post-Retcon, then Franklin's taking this. Franklin at his most powerful has been creating entire universes on a whim and scaring Celestials. I don't think Beyonder will be able to hang
This but if you mean full potential being most powerful then it's an unarguable situation. Franklin's upper limit potential seems to be growing fast as weeds and may one day evolve into a pre-ret beyonder like being as far as I'm concerned. First it was Celestial Level and now it has Celestials sweating bullets.
However at present showings
Pre-ret beyonder 10/10
Post-ret Beyonder 2-3/10 potentially more. I wouldn't count him out unless you want to say he's at his analogue Molecule Man's jobbing levels.
Even Post retcon Beyonder wins, unless it's in the Maker configuration.
As Kosmos the Beyonder was able to achieve universal level reality warping, and as the Beyonder wreaked havok across the multiverse while fighting Molecule Man.
If Pre Retcon this is an absolute stomp in the Beyonders favour.