Well, it depends on a couple of things.
Last time Gandalf should have died he became Super Saiyan Gandalf instead, so he seems basically immortal by the end of the series and almost every conflict is ended by his powers.
Dumbledore has the Elder Wand for the first five or so books, making him unable to be beat. But he loses it and dies in the story, so he is obviously not infallible.
I'd go with Gandalf.