I got curious and did some Googling. I suppose it might be The In-Betweener, The Beyonder, Kubik, or Kosmos. I'm sticking with Living Tribunal though, as I've seen no mentions of any limitation to the magnitude or scope of its power.
The In-Betweener? Didn't he get his ass kicked or tricked by Thanos when Thanos took the Reality gem from The In-Betweener's forehead in his quest to form the Infinity Gauntlet?
Kubik and Kosmos I'm not familiar with, but The Beyonder I am. I remember reading about how he once imprisoned all of the major cosmic entities (The Living Tribunal, Death, Eternity/Infinity, The Celestrials, The Stranger, Galactus, etc.) in some kind of field. I also remember reading how The Beyonder eliminated Death from existence, and that it was Mephisto who convinced The Beyonder to bring her back.
If it wasn't The Living Tribunal's ruling on whether the 6 Infinity gems can be used together, I'd say that whoever possessed the Infinity Gauntlet would be the most powerful, given that The Beyonder is already out of the picture by the time the Infinity Gauntlet came into play. Imagine The Living Tribunal having possession of the Infinity Gauntlet, it wouldn't make sense, but it would be something to behold.
According to the site I've been looking at, Kubik and Kosmos are entities that were born of the same power as The Beyonder. Particularly noteworthy is this part:
Soon, Kubik confronted the Beyonder and, in ferocious battle, Kubik destroyed most of the Beyonder's pet reality and was poised to destroy the Beyonder before the Shaper called him to stop the battle. Under the Shaper's guidance, Kubik, Molecule Man, and the Beyonder realized they shared a common origin, and the Molecule Man and Beyonder joined their power to create a new cosmic cube-- their powers were actually an intelligence like Kubik's that was split during the Molecule Man's origin.