I made an iOS application for reading comics in literally 6 hours. I released it back in May and it's currently earning me ~$1000 a month via ads. Impressions are still increasing by ~25K each day, so profits are likely to increase in the coming months.
Probably manga. The US manga market has expanded hugely since I started reading manga (~8 years ago). Just about any genre (or subject matter) you can name has manga dedicated to it, which contributes to its ability to appeal across all age groups.
The other big thing is that most of the hardcore fans (those who would also like to read on their iOS devices) like to download & read scanlations (A $10 manga volume takes at most 90 minutes to read, so this is an expensive hobby for a hardcore fan, many of whom are children. Also, the hardcore fans prefer the scanlations because they allow them access to manga that hasn't been released in America or has been Americanized by the official English-language translator/publisher.), which are easily placed on an iPhone/iPad, whereas American comics would much more often require legal licensing.
