Taylor Swift wrote every single word on her last album, no co-writers.
Adele/Florence/etc (all the alternative-ish pop stars mentioned on this website) along with Gaga, Madonna, Ke$ha, etc. seem to all write their own songs.
Katy seems like a genuine songwriter to me, but I always notice people like Bonnie McKee in her writing credits, which suggests otherwise. Also Dr. Luke is a songwriter as well.
Beyonce, Rihanna - both seem to have the odd writing credit, but aren't noted for their songwriting abilities.
Britney wrote a bunch of songs from In The Zone and her song Scary from FF too, all of which were pretty good. I have no idea why her label doesn't let her write any more songs