I can see how people say Gaga is trying too hard, and sometimes I agree- with the 10 minute long music videos and the outrageous outfits, etc. but you can't deny the raw talent and the vision.
Katy Perry too, but just image-wise I think. She's always doing some new with her hair and outfits, and it doesn't always work in her favor, imo. Musically, she's always done the dance/pop/rock with I Kissed A Girl/Hot & Cold. All of TD just took that formula and ran with it. Since then, her looks and performances took a turn from her more guitar oriented gigs to a more Gaga-esque (which itself was reminiscent of Madonna, etc) pop-drama flavor with all the choreography, costumes and themed sets, but it all still felt very light-hearted Katy most of the time.
But I vote for Nicki. She came hard onto the scene swinging with the song-stealing features and raps, essentially reviving the female MC, but now all I see is her wearing Gaga's rejected outfits and singing Katy Perry's and Rihanna's rejected pop songs. That Roman Holiday nonsense at the Grammys just ruined it all for me.