For me, double layout on floor. I had at least four other back tumbling skills that were allegedly-harder, but I could never get a double layout no matter how much I worked at it.
As a coach, I think the most common sticking point I've seen is giants. I know kips have a reputation as the major sticking point, but to me a giant is a much more important threshhold and a much more significant barrier.
Flyaway on bars gets an honorable mention. As far as technique, they're trivially easy, but psychologically I think they're the scariest skill in the entire sport to learn.