As of right now, Ariana is on top and shitting on everybody but she also hasn't really had any competition for the past couple of years with Rihanna focusing on makeup, Gaga making more singer-songwriter type stuff, Katy Perry's last album being a flop and Beyonce hasn't really been making pop music since 4, in my opinion.
TBH, I think Rihanna and Beyonce are at a point in their careers where they don't really care about making mainstream pop songs anymore and Gaga is sort of unpredictable because her songs sort of always sound pop no matter what genre she's writing in, her style just naturally leans that way. Then there is Katy Perry, who just seems to be in it for the money at this point and doesn't seem to be too enthused with her own music. Plus there are rumours that Rihanna's next album might be her last in a long time as she plans to focus more on her business empire with her makeup and fashion brands.
The only real competition being posed against Ariana right now is maybe Taylor Swift but even then I feel like Taylor is a lot more divisive from a general public standpoint after all of her public feuds whereas Ariana is almost universally loved after the way in which she handled the Manchester bombings. Then there are a couple of up and comers like Dua Lipa, Camila Cabello & Normani but they're all a LONG WAY off being on Ariana or Taylor's level...