none of them are the antithesis of anything she stands for... Most of them all represent and believe in the same things.
When it comes down to style and aesthetic though, Katy is the most like the "anti-gaga". They just make great rivals. They are very different but also very similar, and their fans hate each other.
I'll go with Taylor Swift. Their image isn't similar, their performances aren't similar, their main selling point is different, they have different musical strengths, Taylor would never act like Gaga and Gaga would never act like Taylor, their musical themes are totally different.
When it comes to what they stand for I would put Rihanna as the antigaga, simply because I don't see as much artistry in her music, just my personal opinion, or at least not as much compared to the others.
However, when it comes to style Adele is the complete opposite to Gaga. Gaga is very flashy and crazy in concert (ex. jumping off an edge while performing 'Edge of Glory'), Adele stands in front of a mic and opens her mouth
I'll go with Taylor Swift. Their image isn't similar, their performances aren't similar, their main selling point is different, they have different musical strengths, Taylor would never act like Gaga and Gaga would never act like Taylor, their musical themes are totally different.