Assuming the atheists are right - Nothing since there's no "me" to miss anything after I'm dead.
Afterlife - I think what I would miss the most is the life I could've had if I was born under better circumstances. Like being born neurotypical, a different time, born rich, a different country, etc. Assuming I CTB'd because I couldn't take this life anymore.
I would also miss the family and friends I've stayed in contact with. I would miss my video games, playing my guitar. Wouldn't miss Reddit lol.