San Francisco and NYC took the top two spots respectively. I'm not sure that the hills of San Francisco make the city the most walkable but it is a beautiful place to visit.
The website is really pretty cool and allows you to see how various neighborhoods ranked for 'walkability'. If you would like to check it out, link here. Back Bay, Beacon Hill and the South End led with the highest scores. You can see how deep green these neighborhoods are in the picture below from the website which indicates it is a 'walker's paradise'.
I would like to add that these people obviously conducted their survey during warmer months, because right now when I am walking to get my coffee paradise is not how I would describe the experience. The children's game "Slip 'n Slide" would be more appropriate.