Based on a number of combined factors -- image, star quality/likability/popularity, talent and the actual music which pop girl would be the hardest to duplicate or find a replacement for? Who is the most stand out, unique and least forgettable of the bunch? Who is the most irreplaceable?
Beyonce, definitely. Notice how her 'flop' album 4 did not affect her star power, celebrity, or status one bit. I think that's a big testimony to this.