I'm also going to go with Lydia. She might not be anywhere near as developed a character as Garrus or Elizabeth or, if we're being perfectly honest, the Companion Cube but I still got a big ol' soft spot for my first adventuring companion, plus she was a huge asset during my character's early levels. Easy on the eyes too.
Also, this: http://www.youtube.com/watch?v=n4zRe_wvJw8
Also, this: http://www.youtube.com/watch?v=n4zRe_wvJw8