Amy Winehouse. Hugely overrated by casual listeners who enjoy her take on something they usually have little knowledge of, or exposure to.
Alexandra Burke. Only overrated on here as far as I can see. A girl who can sing but has no artistic vision. I honestly wish she'd record worthwhile music rather than second-rate generic pap but I won't be surprised if it never happens. If her career implodes, it doesn't matter a jot. Artists with far more impressive track records have fallen right off the radar, without anyone seeming to care or notice.
Madonna. A mediocre singer who survives due to being a package rather than a talent, baring flesh and frequently reinventing herself. I find it laughable that people take her seriously.
Beyoncé. She tries so hard to be an awesome singer that it's cringe-worthy. Dreadful material too. The song she did for Austin Powers was ok but she owes most of her fame to a Chi-Lites sample and her ever-changing appearance/ lack of clothes.