Then that's definitely your opinion. =P There are Japanese bands and singers in all genres, after all.
What I hate about threads like these is that everyone really is just voicing their (usually biased) opinion. "The worst" is just what people think is bad, not who actually can't sing (William Hung aside). Nearly everyone famous is musically talented, or otherwise they wouldn't be earning money. How many people that everyone has actually mentioned has a bad voice, or can't sing in tune, or can't play their instrument?