Lady Gaga because she can't sing
I am by no means a fan of Lady Gaga but she can definitely sing. Have you checked her pre-famous, acapella (sp.) or live performances on YouTube? She has a better voice than most of the popular artists out there today (Britney, Rihanna, Katy Perry etc.). Whether you find her voice appealing or not is of course a matter of personal taste. I do feel that Lady Gaga is not as original or revolutionary as she is made out to be and her music gets more praise than it deserves. I like many of her songs but let's be honest, would they be considered as "genius" if they were sung by Britney or Katy Perry? I don't think so. From the options given in the poll, I therefore find Lady Gaga the most overrated by the media and the general public. That doesn't mean she is not talented.
As for Britney, Rihanna and Taylor Swift, I don't see how anyone could consider them overrated as they are not rated much in the first place. I've never heard anyone calling them or their music legendary or genius. Actually, I think Britney in particular is underestimated as an artist. Her voice may not be great but it's decent, she is a fantastic dancer and she has released some classic songs (like Toxic). Plus she's been around for over a decade, which not many pop artists achieve. She deserves more credit than she gets imo.
Finally, I consider Beyonce to be close to Lady Gaga in terms of being overrated. She has hardly evolved as an artist, neither in image nor music, in the 14+ years she's been around. She just doesn't excite me. I know she has a good voice, she's pretty and she's an alright dancer but I don't understand all the hype around her. Remember her new album that is being likened to "MJ in his prime"? Well, I've heard the first two songs and they sound terrible. Besides, hardly any of her songs carry messages that are in any way original or thought-provoking. The usual subjects are love and girl power. At least Gaga makes an effort. Also, Beyonce has been accused on numerous occasions of taking royalty credits she did not deserve, and afaik she doesn't play any musical instruments unlike Gaga. Then again, Beyonce is not as pretentious as Lady Gaga is. In short, it's a close call between the two but Gaga has the edge for me.