I am not sure, but I would venture a guess battleheart sold well, and it has one beast of an original mechanic. Game play was/is amazing on that one.
You know that game where those slightly angry birds are slingshotting themselves on pigs. Yeah, I know right, who would like that?
This is not an original game mechanic. See: http://www.quora.com/Is-Angry-Birds-a-rip-off-of-Crush-the-Castle
I know... Wasn't the iOS version of Crush the Castle released before flash though? If yes, then AB is kind of original even though it takes ideas from that.