The plot. It doesn't matter if your drawing looks more like a cat scratched it out, if you have a good plot, then you're pretty much set. However, that doesn't mean the other aspects aren't just as important, because if you have a good plot but don't know how to deliver it and make it marketable or if you can't play with expressions accurately, then you won't get very far. Although the art is also important, it's not a deal breaker to me.
So, it's really a mix of all of these points, but the plot is fundamental. The foundation of any good manga or anime, is the plot. Everything else comes after this.