Obviously JT
If you are comparing their entire careers you can't simply leave Nsync out just because he "wasn't the only one!!", by that logic Paul McCartney's career is smaller than some of the girls this site stans for because he wasn't the only one member on The Beatles
JT has had success and hits in 3 different decades and proved he has longevity, Gaga had a huge peak but a fast decline and I don't think she will ever be able to pull off a comeback like JT did in 2013.