My favorite (and most recent) AHA moment was figuring out how to hit a decent kick serve that goes in most of the time. My first goal was to get a hard-to-punish serve and then find the kick serve. Both came at once for me a couple of weeks ago and it's worked ever since.
The one that changed my game was my forehand AHA, though I have hiccups here and there. Consistent topsin plus power equals unstoppable.
I am an unpredictable player. Even I surprise myself with some of the shots I make.