all the people snickering at Britney, we'll see...
during the later part of her career, Marilyn Monroe was ridiculed and regularly lambasted, but people just could never stop talking about her and look where we are today everybody know she a legend.
I think Madonna will always be known as THE Queen of Pop, simply because she was the first, and was a complete trailblazer. That being said, I say Britney and Gaga
These titles in music are honorable, they are given to certain amazing people based on their achievements, legacy, influence and impact. You cant take the title from Madonna, MJ, Elvis, Aretha etc., just because you cant take their achievements from them.
Even when Madonna will die, her music, just like her title will live on.
When there will be someone with bigger impact and achievements than Madonna or anyone out of the music royalty, I believe he/she will get his/her very own title, or we will have more people with the same title. Thats all what can happen.