What is Katy top selling album?

As of my last update in September 2018, Katy Perry's top-selling album is "Teenage Dream", which was released in 2010. The album sold over 1.2 million copies in its first week of release and has since sold over 18 million units worldwide. It features hit singles such as "California Gurls", "Teenage Dream", "Fireworks", and "E.T."

However, please note that there may have been new releases and sales figures since September 2018, so it's best to check official sources for the most up-to-date information.