I found this article online while researching about Minneapolis.
Check out this story.
The top 25 list in pictures can be seen here.
Sorry Ben, but Ottawa is #4, and Minneapolis, #5.
Tokyo is clean, generally, but in parts of Kabukicho and Shibuya, it can get dirty. It seems there is a major lack of public garbages around. I think this is an attempt by the city to prevent people from polluting, it does work to an extent, but some people are just bound to pollute.