Seriously though, Sorcerer Supreme doesn't mean Strange is tasked with an overseer type of role on Earth?
I guess the difference to me is that overseers, like the Principalities, don't just protect but guide mankind. Dr. Strange protects mankind from magical threats, but he's not a guardian in the guidance-sense. Its not his job to guide humanity anywhere. If anything he's often portrayed as not wanting to meddle in "normal" human affairs.
I am of course setting aside his participation in the (Marvel) Illuminati council, which was less Principality and more Star Chamber.
That said, what about the One above All?
That's a weird one. My knowledge of Marvel cosmology is a bit dated, but what I can recall is that from the earliest stories Dr. Strange interacted with a being called Eternity, which was sort of the personification of the entire universe. It is Eternity that sort of validates Stephen Strange's title of being the Sorcerer Supreme of Earth, meaning the dimension that includes Earth, because Earth is always the most important place in the universe. Left unspoken is how one guy can protect an entire universe, ala the Green Lantern problem. If Stephen Strange has a boss, in a sense it is Eternity.
Since Eternity is the personification of this (the Marvel Comics main) dimension (aka Earth-616), other universes could have other personifications. Thus, the question of who is more powerful than all the various Eternities and other universal personifications. Thus, the Living Tribunal was invented to fill that role, as the supreme arbiter of the Marvel multiverse. However, the Tribunal was depicted as a judge: he was introduced as a being that had the power to decide if a reality was worth existing or was too dangerous and had to be destroyed. Because it was a judge (and executioner) it kinda made sense that he would serve a higher purpose or power, and thus it was said he worked for a being above all others - eventually "the one above all others" became known as the One Above All, the ultimate power of all possible realities. Given the cosmological escalation that tends to happen in the Marvel Universe (Galactus, Celestials, Eternity, Living Tribunal, Beyonder, Beyonders, ...) having a being known as "The One Above All" at least prevents anyone from asking the stupid question of who is more powerful than the one above all. By definition, there is nothing and no one more powerful (then again, the Beyonders are one of those stupid decisions that probably seemed cool at the time, except for taking a dump on all previous cosmological continuity, something a lot of comic book writers seem to love to do).
Since the One Above All is the ultimate, ultimate power of all the Marvel multiverse, some say it is essentially God. However, there's no way to prove that, its just said that there is a higher power than the Living Tribunal, who is the ultimate arbiter of existence.
Also, there was the time the Fantastic Four managed to escape all of reality and find the One Above All, and this happened:
The personification of the ultimate power of creation, the only way the mortal minds of the Fantastic Four were able to conceptualize the ultimate creator of all existence, turned out to be Jack Kirby, who is of course the co-creator of the Fantastic Four (along with Stan Lee: the last panel references that).
