The word "not perfect," is that everyone must have a flaw , perhaps with some people might not. "Good," That just means he probably will not. "Best" Yes, but that he is "bad" or "bad" when broken.
The word "is not perfect." Is a defect that everyone must have maybe that someone may not. "Perfect" that just means that he may be. "Best" - but not that he is "bad" or "bad" was broken when.