Ion 4TW
First, I'd like to set a few things straight.
Superman Prime does have a green lantern corps ring, but he does not have the ability to chanell all of Oa's power through it, he would have normal green Lantern abilities in addition to his own power.
Secondly, Ion is more powerful than parallax, and parallax is stronger than even spectre (as stated by DC).
Now, let's look at Ion's Power:
-Ability to bend time, space, and reality.
-Ability to be in many places, and times, at once.
-Ion is one with everything, he is essentially part of the fabric of the universe.
-As a normal green lantern's power is limited by his imagination, Ion's power can extend beyond anything imaginable.
-Because Ion can be in different places and times at once, he is essentially immortal.
Okay, Im not going to touch on Superman Prime's power in depth, but if you read Ion's power description, he would be basically unbeatable....He has Trans-Godlike Powers....As the green lantern powers are limited by imagination, his imagination as Ion is limitless, meaning his powers are infinite and without end, and stretch far beyond human imagination, (or alien for that matter).