Beyonce and Britney shouldn't even be in this question. Beyoncé doesn't even know what a stave looks like. Britney has written too less for their to be an opinion. Taylor and Adele's song writing is too one dimensional. So, Lady gaga is the best songwriter out of the 5.
Bey and Brit are NOT true song-writers. ATRL is so delusional when it comes to song-writing. They play 0 instruments, and know very little about the technical aspects of music.
The others;
Taylor > Adele > Gaga
And you're wrong.
Both Beyoncé and Bey do play instruments and Britney composed a lot of songs on the piano
Britney's not credited as a writer on enough songs for me to properly gauge her abilities. But judging by the songs she is credited on I would place her just below Beyoncé, though still far from the worst.