Can't decide between Gaga or Rihanna...
I remember one time going to Amsterdam and seeing EVERY black girl with red hair, thinking "wow Rihanna had impact". My vote goes to Rihanna, Gaga is close though. Katy not at all
Rihanna because it defined an entire era (LOUD). She was consistent with it and it made her Rihinvention for the LOUD era cohesive. Katy's blue wig was just for 'California Gurls' and Gaga's hair bow just popped up here and there during The Fame era. But neither Katy or Gaga were consistent with the looks. Rihanna was. That red hair reminds everyone of her fifth album, and she'll never bring it back because it represents LOUD and LOUD only.
Gaga's bow is recognizable, but it didn't have impact beyond being a Lady Gaga prop.
Rihanna's red hair was also recognizable and thousands of girls were getting the same color and haircut, it got beyond 'hey that's Rihanna's hair'.