I liked all of the standard female companions (sorry, Kasumi doesn't count) for providing a different angle on what I see as traditional video game sexuality (Lara Croft, Dead or Alive, etc.)
Miranda was completely aware that she was designed to be an impossibly hot piece of ass, and feels insecure about her own worth as a result.
Jack is the most overtly sexual member of the team, but it's because of her life of physical and sexual abuse and nihilistic attitude towards other people.
Samara is a gigantic tease, wearing a completely impractical suit of cleavage-tastic armor and letting the player get right to the brink of doing the nasty before refusing. (Alternately, her daughter will sex you to death.)
Tali is the most literal example of "It's what's on the inside that counts" ever to grace video games, with few discernible facial features, zero exposed skin and shy-nerdy demeanor.