A person who constantly feels obligated to follow a story you tell, with a worse one, making them sound like they've been through a much more terrible ordeal.
Susan: Today I slipped on the ice and cut my head open.
Tom: Yeah well, last night I slipped on the ice, broke two ribs, cracked my head open, AND my girlfriend broke up with me.
Susan: You're a story topper. It's lame.
When someone is tellin a story and another person always comes and makes it better than it really is by tellin a different story.
Bob:So i was driving along the road and saw so ducks so i decided to go shoot them on the side of the road.
Billy: O yeah well i saw ducks to but i decided to drive off road and then through the pond jus to scare them off and shoot them.
Bob: Billy you are such a story-topper.