Well this from O. Westin’s Micro SF/F Mastodon account is the most affecting 323-character sci-fi story I’ve ever seen.

I grant you one wish," the magic fish said.

“To eat my mother’s pancakes again, just like when I was a child.”

“Are you sure?”

“You think it frivolous?”

“No. But to grant it I must send you back to the child you were, with no adult memories.”

“Would my life change?”

“You ask that every time.”

(h/t Robb Knight)

Admittedly it’s the only sci-fi story I can ever remember seeing with 323 characters - but even if it wasn’t I think it’d probably still be top.

In other short stories that had a vivid emotional impact on me, I’d recommend David Foster Wallace’s “Incarnations of Burned Children”. If you don’t mind extremely grim stories (its title isn’t misleading). That took up my brainspace for a real long while.