There's a lot so I'll just list them:
Britney Spears - Blackout
Different sound for its time.
Rihanna - Rated R
Not a lot of mainstream material on it.
Beyoncé - 4
Not a lot of mainstream stuff either.
Madonna - Ray of Light, Music, and Erotica
All three had music and imagery that was from the ordinary.
Lady Gaga - Born This Way and ARTPOP
Born This Way referenced religion and other touchy topics. ARTPOP was released right when the EDM trend ended and its concept was difficult to grasp. Born This Way and ARTPOP, sonically speaking, should have switched release dates.
Christina Aguilera - Bionic
Clearly ahead of its time.