A ton of apps... I guess I most regret buying Asphalt 4, Ferrari GT, Fast And Furious: The Game, Raging Thunder, and GTS. I prefer Fastlane so much more to those racers.
I've also bought a lot of apps hoping they would help me out in a roundabout way only to find a month later an app updates or a new app comes out that does almost exactly what I want it to do and is just so much easier to use.
I guess some apps that I dropped in favor of another I still keep tabs on cause, well I've already bought them, and a few updates later that app could be so much better. That especially goes for personal finance apps and productivity apps. For example, I use TapExpense now cause entering on the fly is so fast and easy with that app, but other apps are getting good as well, such as PocketMoney.