Zombies. Pretty much any media featuring zombies is either actively made worse by their presence, or they bring spectacularly little in the way of interest to it. Even straight up zombie films suffer from this imo. Shaun of the Dead is one of my favourite films, brilliant in its opening act, yet the moment it becomes a more overt zombie film it steps down in quality immediately. Another example, Red Dead Redemption, my favourite game of last year, has a zombie apocalypse themed add-on. It's widely regarded as one of console gaming's best add-ons ever, and yet the zombies succeed in only making the gameplay more annoying and reducing the vanilla game's charm.