If you're talking about impacts on my life then the storm of July 25th/26th 1985 is right up there as I was only 10 and had seen nothing like it before. Other homegrown honourable mentions are "The Big Snow" of January 1982 and ex-Charley of August 1986, and more recently the snows of 2010 and 2018.
Abroad, the thunderstorms in Houston in August, 1992. I hadn't witnessed this type of tropical activity before. I distinctly remember one Saturday evening being driven out to a rodeo in some small isolated Texas village, mouth open watching the anvil-crawler lightning out ahead, each flash lasting for what seemed like seconds. Other storms when I was there had torrential downpours that generated flashfloods in minutes, which would then dry up equally fast as if nothing happened. One such storm apparently dropped a tornado about a mile away, though I didn't get to witness it.
Another was flying from Rome to Bangkok, June 2002. Thunderstorm over northern India, and me like an idiot head stuck to the window watching the strobe light show for around 30 minutes non-stop as we flew around it to its south.