I'd say Cannabis. But I didn't really "overcome" that one, it just got to the point where it was just time to leave it behind. So really, despite my plans to "try" and "quit" smoking it, it sort of happened of its own accord. I think that tends to be the way with these things for me though. I binge for a while, whether it's a week, a month, a year, more, and then the spark dies and I can move on.
But yeah, feels great no longer being dependant on it to feel normal, although I still can't sleep worth a damn, not that I ever could, and it's the only thing that ever worked at getting me to sleep with any consistency.