Are the ones that you never planed on making. You spend all your time getting good at making the little decisions a little faster, a little smarter and a little more often. But you never get to practice those impossibly hard decisions that could make or break you. That's why they are the hardest.
You can't anticipate the hardest circumstances in life. Sure, finishing school, that huge project and the all-nighters are hard...but they are times we endure and succeed. Circumstances that require you to muscle through are never as hard as the choices you make about which life-turn to take at a fork in the road.
The good part is that these hardest decisions often bring you to places you never imagined you wanted to go. Perhaps they end up to be better places than that place all your little decisions were supposed to take you.