Liz was second only to Grace Holloway in terms of sexiness, in my opinion. Intelligence, self knowledge and confidence makes for a sexy personality and they were both physically attractive as well, I've always thought that in Modern Who, that's the sort of character they should have chosen if they wanted us to think that an ancient alien genius could realistically be attracted to her.
Grace was my first Companion. stranded between Classic and Modern Who and she struck the young me as the epitome of sex appeal and beauty. On the other hand, as she was more mature and of a different time, she'd never wear a mini skirt, so maybe Liz (who I haven't seen too much of*) has it...
(I'm comparing like with like here, so no female Time Lords, though Daphne Thingy would have given even Mary Tamm (?) a run for her money.)
*When I say I haven't seen too much of Liz, I mean in terms of her stories, not in a Jo Grant/Dalek way