Can anyone defend the Ontological Argument against Kant's criticism that existence is not a predicate?
Sure. Even if existence is not a predicate, it's at least arguable that necessary existence is. (As Norman Malcolm pointed out years ago, there really are two versions of the argument, and the second one deals with necessary existence.) We doubt that existence is a predicate because, roughly, saying that something exists tells us nothing about what it's like. Not so for necessary existence. Not just anything could exist necessarily. The computer I'm typing on is the wrong sort of thing to be a candidate for necessarily existing thing. Assuming that some things are of the right sort to exist necessarily, necessary existence would be a predicate. Whether this is a defense of the argument all things considered is another matter. But I think the point made here is fair as far as it goes. A being that merely happened to exist wouldn't be a being than which none greater can be conceived.