Easily Lady Gaga.
People calling Gaga's music "outdated" are like, so dumb. How can music that is heavily based/ influenced around nostalgia and a certain time period be "oudated"? That's like saying a modern jazz album is "outdated" because it is influenced by the 30s/ 40s. Like... the people who say that make 0 sense.
While I think Katy's music is decent, I dont think her music will stand out in 50 years time. It's not really classic or anything. Just singles that are a product of the time. Not innovative or that distinctive. While, I think Gaga is anything but this. Thats just my

though.