No he's not.
There are several cosmic beings.
Galactus is one of them.
But Eternity and Death are far superior to him.
And Death is, in some ways, superior to Eternity.
Eternity and Infinity are equals, with the Living Tribunal above them.
Then there's beings like the Stranger, Hate, Love, In-Betweener, and several others.
Galactus isn't all that "powerful" compared to other Cosmic Beings.