Penn - Best curriculum, city location and good geographic location for me, international opportunities, children's hospital, etc. etc. etc. etc. etc., really I could go on forever. Sadly, I am waitlisted.
Stanford - I kind of applied here as a "game" because I heard they liked to reject people fast...my friends and I wanted to see how far we could get? Then I got an interview, actually read about the school, and absolutely loved it when I interviewed, now I'm just about as enthused with their program as I am with Penn's.
Columbia - Everybody makes mistakes...I originally applied b/c i thought living in New York would be cool and I heard they were big fans of the brain. I like the brain, too.
I also applied to Duke and Yale on my AMCAS application. Then I realized how painful the Duke secondary and the year of research would be and ditched Duke. Some MD/PhD kid in my lab told me all about the trouble several of his friends who graduated from Yale Med were having with residencies (not getting in, but once they were there), so bye bye Yale.
I would have applied to Harvard, but the thought of taking more calc to meet their math requirement wasn't worth it to me.