Good match.
Molecule Man wins this. (but it could be an interesting battle)
Owen Reece,
is one of the most powerful beings in all of Marvel's creation according to the LT,
and that's Owen (the limited side of MM) ouch.
He proved it too.
He actually ripped the essence of the Beyonder out of Kosmos.
When he stomped the Beyonder,
reality in Realities across all of Marvel's creation (Omniverse) were warped.
Abraxas is an absolute Multiversal power,
but as far as I know, he's not beyond Multiversal,
while MM proved on panel,
his power reaches beyond the Multiverse. (simultaneously)
In fact, according to Kubik, no mere entity made of power,
(like Cube beings or Concepts)
can ever hope to defeat MM.
Still though, imo, MM wins,
but after an incredible battle.