Gaga at least has a great voice. Can't say the same about Madonna, and surely not about Britney.
Madonna was great up until she became more about the image and celebrity than the music releases. Her 80s work was fantastic, standing out as great among many other great and worthy contenders of 80s pop.
Brittney was...eh. From a nostalgic point of view I like her because we're the same age and she was mega popular when I was in HS. I never cared for her actual music much, but it was catchy enough.
I couldn't tell you about Gaga. I've heard like three songs, only knew one was hers for sure at the time because the song was so big. I wasn't horribly impressed, but that's as much a decry of formulaic songwriting as it is a knock on her.