Fullmetal was rated in the very top but them it was completed. It's still one of the best and everyone knows about it.
One piece has earned it's top spot fair and square and really is one of the best out there right now.
Naruto however is way beyond overrated. The manga starts out pretty good and goes down hill from there. The fame probably comes from the games and such which I have heard should be pretty good.
Bleach lost it's plot somewhere but people seem to fail in realizing that.
I do think Naruto and Bleach have been dragged on for to long with such a basic plot but calling it overrated is a bit rude. Or maybe it's the truth.
Bottomline is: Don't group them together just because they are all famous. One Piece happens to be an epic manga but people judge it too quickly because it's so mainstream.