For London it's the 2km long Oxford St -560,000 visitors per day (and infamous for overcrowding) with over 300 shops having a front on it (and numerous more in shopping centres and side streets).
It's not the most beautiful, but most popular.
www.kevinallen.photodeck.com
100,000 a day pass the crosswalk at it's centre
https://ichef.bbci.co.uk
http://islingtontribune.com/article/...-for-christmas
At Xmas for a select few days it becomes pedestrianised (normally taxis and buses are allowed down it)
It also happens to be,
possibly, the world's most dangerous street. The high crowds on pavements too narrow to accommodate them (historic London zoning meant human-scaled streets throughout, in contrast with Paris or even Manchester) means the city's highest incidents of human-vehicle collision. However what's worse is it has the title of the highest air pollution ever measured in the world. This thanks to NO2 from the buses -the world's highest hourly and annual levels, breaking the global record at 463mcg per sq metre on one measurement (and the average of 135mcg for London, the world's worst performer -by comparison Delhi and Mumbai are at 62mcg). It exceeded the EU
annual safety level in only the first 2 days of the year -air pollution kills multiple times more than traffic accidents, about 10,000 a year for a city the size of London (and that's
with congestion charging).
If ever there was a case of pedestrianisation needed anywhere in the world, this is it:
www.telegraph.co.uk
The Mayor, Sadiq Khan has been battling for years now to pedestrianise it but local residents (read: the notoriously conservative Westminster Council) have repeatedly vetoed the plans.
Recently however, as retail has crashed around the world due to online shopping, plus the opening of 2 vast mega-malls at the edge of the Inner City, the crowds have thinned. The case of pedestrianisation has come far too late.