It's funny how the Adele stans come running into Gaga threads to make it about Adele. She is not a pop star. And even if she was-- she has one massively successful album, yes, but there's no proof she won't go the way of Norah Jones or Alanis. Except Adele would be lucky to have even half the talent, creativity, and quality of music that Norah has
This thread was clearly asking about actual POP STARS, and Gaga's main competition is Rihanna. Her maintained relevancy since 2005 is pretty impressive, but I'm not sure if she would be able to do it without yearly releases and toooons of singles. It's hard to compare someone with 2/3 albums and a handful of singles to someone with 7 albums and dozens of singles (plenty of which either have Just Dance success, or MTN floppage).
Overall I'd say Rihanna has (some) better numbers, but as a result of being in the game for way longer. Personally I think she would have an absolutely huge GGGB-style era again if she took a few years off, and then released a great album that people would be dying to hear. That might really win her the title of "current queen" or whatever you like to call it
Assuming ARTPOP doesn't flop, then it would seem Gaga is the most successful current pop star.