My favorite is the em dash. It says, "Stop! Look at me. Take notice. Something important is happening here." To my mind, it's the prima donna of punctuation, in the best sense.
And then there's em's bastard cousin, the hyphen. Half the size, twice the trouble! It intends to clarify, to help avoid confusion. "The road to Hell is paved with good intentions." I'd like to commit punctuational genocide on the hyphen.