depends on which sauron it is. If he has a physical body (before having the ring sliced off) then spawn wins with EXTREME ease. If it is after he has lost his pysical body (when he is shown represented by "the eye"😉 if he was to get the ring back, no force in heaven, hell, or earth would stand a chance.
Sauron would gain Physical form once retaining the ring again. Its says in ' The Lord of the Rings' (the book). Yes he will become extremly powerful after retaining the ring, just not as powerful as you say. He was a servant to Morgoth and Morgoth was defeated. So Sauron isnt invincible.
My money would be on Spawn.
you fools Sauron was stronger before he made the one ring. infact the ring is intself his weakness as he plundered so mutch of his strengh and he thus became weakner and crippled ( because he carnt die)
if you read the silmeralion it will tell you how powerfull a sourcer was how he was able to change from one dark fell form to enougher inculding vampires and warewolfs