I'm not entirely sure specifically what that particular 'cyanide and happiness reaction fanfic' is about without reading it. It could be a fan - created story based on the 'Cyanide & Happiness' series that focuses on the reactions of the characters in different scenarios, perhaps some really wacky or dark humor - filled situations that are typical of the original content.
You could try looking on fanfiction websites. Websites like Archive of Our Own or FanFiction.net might have it. Just search for 'cyanide and happiness reaction fanfic' in their search bars.
One interesting 'One Punch Man Reaction Fanfiction' could be where other heroes react to Saitama's true strength. For example, they might initially be skeptical of his claims but then are completely astonished when they see him defeat a powerful monster with just one punch. It's a fun exploration of how the other characters in the 'One Punch Man' universe perceive Saitama's overpowered nature.
I've heard that 'The World's Reaction to One - Punch Man' is quite good. It delves into not only the reactions of the characters directly related to Saitama but also how the general public and the media in the One Punch Man world respond to his presence and actions.
One interesting aspect could be the diverse reactions of different characters. For example, when Saitama does his usual one - punch move, how other heroes or villains react in the fanfic can be really engaging. It might show new perspectives on his overpowering strength.