May not be the most impressive, but I may have the most unusual sports accomplishment on this board.
In HS (I know, I know) I struck out a midget in 4 pitches.
Not exaggerating his size, he was at most 4' tall, and he then crouched in the batters box, coming in late in the game as a pinch hitter. Did not swing nor intend to. After the game his coach said that was the first time ever he was not walked.
May also have the least impressive.
My last pitch in HS (district tournament) was a HR ball, and the same hitter also started the inning w/ a HR off of me. Yes, gave up 2 HR to the same batter in the same inning.