She still had some sort of involvement. And you don't know what goes on in the studio and how many words she wrote so bye with that.

And she isn't listed last on the songs she co-wrote on TTT fyi.

Live Vocals (Ballads):
Rihanna > Katy > Nicki > Ke$ha

Live Vocals (Midtempo/Uptempo):
Nicki > Ke$ha > Katy > Rihanna

Songwriting:
Ke$ha > Katy > Nicki > Rihanna

Ability to Play Instruments:
Katy > Ke$ha > Rihanna > Nicki

Dancing:
Rihanna > Nicki > Ke$ha > Katy

Stage Presence:
Ke$ha > Nicki > Rihanna > Katy

Overall:
Ke$ha > Nicki > Katy = Rihanna

Just my opinion
I do enjoy watching/listening to them and I appreciate their talents.

All of them have some level of talent otherwise they wouldn't be famous but in my opinion it goes
Ke$ha = Katy > Nicki > Rihanna

I Think Both Ke$ha and Katy are decent song writers and have the potential to be really good live:

@4:05 onwards

I have no idea
this is probably the hardest poll ever
I guess it's Ke$ha