I thought that "the powers that be" u know LT down to Celestial allowed the beyonder go through this whole 'I'm the greatest" thing because they knew the out come. And like it's been said before he was only the uncontained portion of the cosmic cube without imposed limitations....As for all that him being from beyond the multi verse crap that is definitely svfl if I ever saw it. I think the guys at marvel knew it and tried to clean it up.
I feel it's the main thing which makes Marvel better than DC, the fact that there are definite limits to characters and structure to the total fabric beginning with TOAA. The whole recon beyonder crap threatened to unravels decades of continuity. In fact his character is a big reason I stopped buying comics for bout 10 years 😠 ( ahhh! got that off my chest)
The Marvel character who has displayed the greatest level of power on panel would be the Phoenix. However the character who appears to have jurisdiction over all is the Living Tribunal. In terms of presentation the Pre Retcon Beyonder was the most powerful however regardless of all the talk and hype he had not a single feat beyond the likes of the IG, Heart Of The Infinite or the Phoenix.