Here we can talk about character's whose broken power-level is what makes them, well, them & if they are watered down, they are basically not them anymore (like Alien X but he can be K.O. by being hit with a car).
I know it is possible to water down powerful characters respectfully but any watering down would be = to making Superman a human powered by magic.
Here's my list:
Nate Grey/X-Man
Knull
Blue Marvel
Anyone who possesses the Power Cosmic
The Sentry
Hyperion
The Beyonder
What are you guys' list?