A-List is one of the hottest right now, slaying in all aspects, etc. So yeah, Katy, Bey, and Taylor are A-List.
Gaga is B-List with Rihanna. They're not the hottest right now or slaying, but they're still relevant and not having albums struggle to go Gold or singles go T40 and just nostalgia at this point like Britney & Christina.
Ke$ha was a one time thing, has no following, has no relevance, and isn't really remembered. That's D-List, along with LMFAO and Ashanti.