What is the hardest to do in a game?

Perfect game hands down. The others are up there, but to have a clean sheet for 9 innings is nearly impossible. Since pitchers begin to tire as the pitch count goes up, since batters eventually pick up on pitchers and take advantage of their exhaustion in the later innings along with your defense perhaps botching a fly ball or a ground ball it makes it difficult to be perfect for 9 innings and 27 straight batters.