Team, Surfer on his own might win a few via astral plane attacks.
Onslaught was depicted as being at "Full Potential" when he fought (and lost against) Hulk. he was feeding off Nate and Franklin as well as having all of Magneto & Xavier's psychic energies.
as for the Celestials thing... perhaps he would have been at that level had Franklin been at "full potential", but nothing Onslaught did indicates anything beyond Skyfather.