The Phoenix Force has been referenced, in-canon, as the original spark that ignited the universe and the eventual fire that will consume it. Reed Richards himself postulated that this exact energy, the Big Bang energy or what have you, is what originally gave Galactus his being and existence in the first place. Yes, the most pedestrian and straightforward way of describing the Phoenix Force would go something like "the combined essence of all psionic energy in the universe," but it's so much more than that. Life in the Marvel universe, by definition, is psionic energy. When the Phoenix says "I am fire! And life incarnate!", it's not just a catchy slogan; she's literally describing what she represents.
And yet on that note, she doesn't just represent creation but destruction also, since it's her job to burn away what doesn't work, thereby ensuring that life evolves naturally as it should and "fixing" the universe. For the potential of creation to exist, there must first be destruction. Just as the mythological phoenix, the Phoenix Force represents the cycle of life in the universe, that of death and rebirth from the ashes. And just as this universe ends, another will be born from the ashes. And make no mistake; her power level has been described by the Watcher as to be second only to the Creator. We've seen her hold the entire universe in her hands, after all.
(However, it should be noted that the Phoenix has very rarely displayed that level of power. The sort of universe-bending potential presumably requires an incredibly stable and harmonious balance between the Force and its avatar Jean Grey...something that, admittedly, almost never actually happens.)
(And I think that the Living Tribunal is more powerful, anyway. Or at least he's supposed to be)