Usually the most fun and successful iPad apps are ones which are best suited for a large touchscreen. Apps such as Flipboard and Paper are successful because the only place such apps work best are on the iPad.
Blek is a new iPad game that would only really be suited best on an iPad. The aim of the game is to use gestures to form a line that loops infinitely. That line has to be drawn in a way in which it can hit all the coloured blobs. But you have to avoid the black blobs, they're the ones which make you start over.
The game is very minimal in design and play but boy is it addictive. Once you get past the a levels, you're put straight into the next one - no hassle, no waiting around, just straight back into it. It makes you want to go as far as you can as quickly as you can.
Of course as you progress levels get harder. They make you think a lot and try all sorts of line combinations to see what will work.
The game is currently available for iPad, but the devs have told me the iPhone version should be in the App Store in the next 12 hours.