Anslem has been refuted so many times in antiquity and in modern times.
One objection is that he is guilty of equivocation. A real god is not greater than an imaginary one it is a completely different class of thing. He slips between one definition of the word god and a totally different definition.
Another objection is that the argument doesn't get us to the god of Christian theism. The sort who is active in the physical world.
Thirdly something that can be argued for through deductive argument is useless unless it can then be shown to actually exist in reality.
These are only three very brief objections. Many more detailed refutations can be found with a little research.