This is in order:
1. Ke$ha (simply because once this pop/dance/electro sound is over, she's over)
2. Katy Perry (i just don't think she has enough appeal to last longer)
3. Lady GaGa (she has built up a big fan base in a short amount of time, so I think she has nothing to worry about for now, unless a new gay star comes along and steals attention)
And RIHANNA let's just not go there, she has proven to survive the past 5 years, while others came and left!!