A Celestial!
In Guardians of the Galaxy Vol. 2, they established that Ego is both Star-Lord’s biological father and as a living member of the Celestials. In the comic books, the Celestials were an all-mighty race that went extinct, save for Ego, who protected and were capable of wielding the Infinity Stones’ power.
However, the movie doesn’t make much mention of Celestial beings other than Ego, but we do get a good look at one. For a brief moment, Eson the Searcher is seen on old footage, which shows him destroying a world with the Power Stone. Theoretically, that should’ve made him impossible to beat, but he relinquished the stone for some unknown reason.