Ashley Williams.
I like strong independant women, not to mention she was quite a well written and believable character. She had flaws, no question, she had prejudices against aliens due to her family's but if you pursued the right dialouge with her, her opinions actually change on the matter of aliens.
Besides she was hot (while still looking believable and not being used as eye candy) and I have a thing for tough chicks with laser guns and given the choice I happily chose her over all the others because the chemistry between her and Shepard was both funny and at times kind of sweet, without getting cheesy.
Sure, the confrontation on Horizon in Mass Effect 2 put a damper on things, but combined with an apologetic email and the realisation that I couldn't really blame her for reacting the way she did, made me decide to pass up on Miranda, Jack and Tali and hold out for more Ashley and Shepard in Mass Effect 3. I liked the idea of a faithful Shepard better I guess.
So yeah, I chose Ashley for the first and I stuck by my choice in the second. So in the end Shepard merely gazed longingly at her picture.