I've always had my greatest success when I scratched a personal itch.

My first released app, iRooster, helped me wake up in time for class, and ended up paying my rent after Apple featured it in a weekly email blast.

My first iPhone app helped me track the daily fluctuations in electoral vote counts between Obama and McCain last year. In the first two weeks, I catapulted into the top 10 paid apps list on the AppStore, spent hours on conference calls with a fair number of bigwigs at Washington Post, and received a very nice writeup from Walt Mossberg.

YMMV, of course, but scratching an itch means you're going to be able to solve a (relatively) real-world problem, probably help other people out, and will definitely increase your long term happiness.

Good luck!