Hmm....oh my gosh, there's too many!
But if I have to choose, I'd choose "White Houses" because it was the song that made me re-discover her.
After "A Thousand Miles," I'd actually forgotten about her *gasps*. I remember, back in '07 (I think), I googled her name, came up with her website. My reaction: Oh, really? Her new album's called "Heroes & Thieves"? The next day, I forgot her name. Again.
I remembered the name last summer. I googled a random word which just so happened to be "harmonium" and came up with "Harmonium is the second album of Vanessa Carlton, etc..." I remembered her name and picked a random phrase from the article. I found it in the tracklist, and it was "White Houses" and it all goes downhill from there.
Too long? Sorry.