Leia has to take it - if only because she is the only one so far to have appeared in a bikini. Also, as jpowers said, she had such good dialogue. If a bad hairstylist.
I am a big fan of Amidala myself, and of Natalie Portman (though she's been going downhill since Leon). She is also a mean shot with a blaster (obviously runs in the family) and has the most prominent nipples of the three (someone was reported as saying when they came out of Attack of the Clones 'You could hang a wet duffle coat off them').
Arwen is just some fey bit of rubbish put in to make up for the fact Tolkien wasn't writing for women. I haven't seen Two Towers, but there is nothing in the first film to commend her to me. Liv Tyler, on the other hand, can use my shower any time.