Rihanna easily. She had FOUR #1s in a CALENDAR YEAR (the ONLY solo artist to do it and went down in the record books for it). Her album Loud was uber successful and was platinum by January 2011 (it had only come out mid-November 2010). None other could compete with her that year other than Taylor Swift but Speak Now's single success <<<<<<
Gaga got 'em album sales that year, three singles that effortlessly globally slayed all the pop girls eras, her fanbase grew super huge thanks to the era.
The critical acclaim,the impact was felt globally her tour smashing all the pop girls yet she was a newbie,the awards came rushing in,the grammy awards anyone.
I did not see Katy,Adele or Rihanna doing that.
What impact? She had impact with Bad Romance, but it was overshadowed by TiK ToK.
Overshadowed? How many views has TT on youtube, how many grammys has TT, how many VMA's has TT, how many EMA's has TT,??
Telephone has sold more than 7M , Alejandro- more than 5M .And Alejandro was the summer hit of 2010.
What impact? She had impact with Bad Romance, but it was overshadowed by TiK ToK.