As
>>3 said, it is entirely subjective
My opinion is as follows:
If by good, you mean well designed, then a good book character is one whose existence and development are essential to the reader's understanding of the story's plots, themes and messages, within set limits of (ir)rationality dictated by the author. The effectiveness with which the author uses/fleshes out such a character (and all their interactions)would dictate how good a character it is.
If by good, you mean "to be used as an example during a character creation workshop or during moralistic debates," then a character is good when they exhibit qualities which are deemed "good" and previously praised/immortalized by any given society at any given time of human history.