Answer: Publish Pacman.
At launch, Pacman was the top game on Facebook and remained so until the rights owner Namco sued to have it taken down.
As a widget on Myspace (before they had apps), Pacman was the most popular game widget.
And now, on Android, Pacman is the top game according to Medialets.
Alas, I couldn't find any data on Pacman for the Iphone, but I'm guessing it did pretty well.
So what accounts for the success? I think it comes down to the power of familiarity. Everybody knows Pacman, while at 94% of Americans do according to the Davie Brown Brand Awareness Index.
Think about it. If you're looking through a long list of games then you're naturally going to pick the game that you know and like. Of course, whether you play it more than once is an open question...