I would say Gandalf.
Dumbledore's magic is obviously powerful and impressive, and my opinion might change with the Elder Wand in play, but even with it, it is clear that Dumbledore is operating at a mortal level while Gandalf is a divine being.
One could argue that Dumbledore's mystical effects are more overtly powerful in magnitude and scale, but Gandalf does not need to be overt: He is that powerful.