The sun's radiation causing the Earth's core to collapse or some shit. 2012? The Core?
A man who's had his heart removed has to constantly electrocute himself to stay alive. Crank, which was awesome...
When giant monsters attack we spend untold trillions of dollars to create gigantic robots to fight them in hand-to-hand combat (and get angst-ridden borderline psychotic teenagers to pilot them). First thought was Pacific Rim until I read 'teenagers'. Neon Genesis Evangelion.
A group of survivors get stranded on an island and after 6 seasons it turns out they were called there to stop Satan. Lost, which again was awesome...
The entire galaxy's greatest protector of peace is an ancient order of warrior monks who abduct children to their services at a very young age to forever live a life of unquestioning servitude, devoid of free will, emotion or sex. If you do any of those things, you get labeled as a heretic and a traitor.
Aliens whose only weakness is water attacking a planet that is 70% water inhabited by creatures whose bodies are 65-something % water. Signs.
When a ghost civilization has grown tired of existing only as a memory they bring a sports celebrity alive from themselves to kill the planet-terrorizing ancient monster that's serving as their life support.
A time traveler fights every monster you dreamed up when you were 10 years old, including ghosts that are actually aliens who create zombies to make their civilization survive.