Uh, hello? Obviously Gandalf? :P As The Owl of Minerva said, Gandalf is a Maia. The Maiar came down to Eä (the plane of existence in which Arda, the planet containing Middle Earth, resides) at the beginning of time with all the other Ainur. They are less powerful versions of the Valar, who are literally gods. Galadriel's wielding of Nenya, the Ring of Water, renders her more powerful than your bog-standard elf, but Gandalf has the Ring of Fire and magic and is a god-like being.
In fact, when Gandalf the Grey died, it was Eru Ilúvatar who resurrected him - Eru Ilúvatar who stands outside of time as the all-powerful supreme being and created the whole universe.
In part it depends at which point they are fighting. After the destruction of the One Ring, the other rings of power lose their charm, thus eliminating Galadriel's. However, regardless I should think that Gandalf would win after his resurrection.