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.