Opera Mini No. 1 in the AppStore in almost all countries
Link to current statistics - www.apple.com/euro/itunes/charts/apps/top10appstorefree.html
1 million times downloaded on the first day.
For reference: the widely expected Skype for iPhone was downloaded 1 million times in the first two days after appearing in the AppStore, and it took almost three weeks for the PayPal application to overcome the millionth installation, while having 81 million army of regular users.