Over all the years and comic books, which comic book story snuck up and grabbed you, and it has to be a one shot/stand alone story. Ill list mine as examples.
Amazing Spider-Man #248 "The Kid Who Collects Spider-Man"
This story exemplified why Spider-Man is probably the greatest superhero in comic books. His humanity. I was so taken by surprise by this issue Id have to think its why it had such a great effect on me, and might not on you if you havent read it. When Spider-Man comes to this boy in the middle of the night (in Spidey style), and just lays his heart out to the kid, and answering any question asked. The whole time youre wondering "Why is he doing this?!". Then you find out.
Fantastic Four V1 #605 "End of Line"
A Jonathan Hickman standalone masterpiece. Reed Richards and his father Nathaniel travel into the future because Reeds in one of those "I just gotta know" type moods. To anybody that ever like The Thing / Ben Grimm, this is a must read.
Thor V1 #362 "He stood alone at Gjallerbru"
Thor and the Asgardians have just finished invading hel (no I didnt mispell it. Thats how the Norse spell Hell), and are having to get the he..., well get out of there. This is alot easier said than done. Grab a hanky.