Well, you have Jean Grey, who was essentially The Phoenix Force Reborn into Human Form, as the "Ultimate Mutation."
Rachel Summers, being her daughter, essentially possesses the same relationship with the Phoenix Force, and the same level of power and potential.
When Nate Grey came to Earth, his Psionic Energy was the most powerful that Xavier had ever sensed. So powerful, his body could barely contain it.
Cable possesses the same potential as X-Man.
The Celestials acknowledge Franklin Richards as the ultimate creation and end result of their experiments on Earth.
Hyperstorm, being the offspring of both Rachel Summers and Franklin Richards, would logically possess extraordinary potential. In one Universe, he was estimated at being at a level of power of that of Magneto, Xavier, and the Phoenix Force combined.
Apocalypse could very well possess the very same techno-organic virus that Cable does, yet his power is still phenomenal.
Proteus was suggested as being possibly the most powerful being in Creation, and possessed the power to destroy the entire Omniverse.
Legion possessed 10 times the psionic power that Xavier did.
Jaspers and Braddock have displayed phenomenal abilities with their mutant powers.
I feel the above deserve major props.