Mohawk Storm
Uncanny X-Men #173, October 1983 - the issue where Storm changed her look. Gone was the long-haired, cape-wearing woman from Africa - she was now a leather and Mohawk wearing radical from the 80's punk scene.
It was a pretty bold change at the time - giving the previously happy and optimistic Storm a rebellious and darker attitude. But it's that mohawk that makes the change memorable.