GaGa pouts a lot and her lips are always plumped up. I'd say she's average at best but she definitely gives really good poses and knows how to work'em lips. Beyonce has natural sexy lips but does nothing with'em.. Britney has a cute smile but that's about it. Rihanna has really weird-looking lips.. They're like two Illuminati triangles on top and full but not in a sexy way. Katy's lips are too average. Christina's lips are average but she is the queen of lipstick and gives spectacular sexy poses. Miley's upper teeth ruin everything for her lips.
I love Rih's lips, specially the lovely shape, very sexy. I really like Lady's lips on pink, I wonder if that's their natural shade because it's gorgeous.
For me it's Beyoncé (seems obvious) but I think that hers are really the best. They're full, round, soft and sensual without being too much. Rihanna is followed close behind. But because of the angularity to hers, I find them less "sexy" in the full sense of the word and more beautiful. Gaga's are also really nice and have a lot of personality.