In no particular order, I bring to your attention the 5 best flashmobs on youtube.
Frozen Grand Central
One of the most famous and most watched flashmobs on youtube (so far over 18m views). This one was pulled off by the pioneers of of the modern flashmob, Improv Everywhere, in Grand Central Station, New York (click link to see more of Improv Everywhere's stunts and mobs).
The T-mobile Dance
Another flashmob with over 10m views on youtube and one of the best viral marketing campaigns I know of. This one was organised by T-mobile in the UK and pulled off in Liverpool Street Station (I guess train stations are the in-place for flashmobs?).
Latvian Folk Dance
Not many views on this one, but it was only done a few days ago (20 August 2009), so I'm sure it'll go viral pretty soon! This one features a birds-eye-view shot on the mob which makes it look even cooler. Fancy.
Michael Jackson Tribute
There are many MJ Tribute flashmobs in different locations around the world floating about youtube. This is one that happened in Stockholm, Sweden. It's not strictly a flashmob due to the apparent forewarning, judging by the gathered audience, but still pretty sweet!
The Sound of Music with a Twist
I believe this one's in the Netherlands, can't really tell... but awesome none the less!
Anyone have a better flashmob and wish to prove me wrong? Post the URL in the comments below!