The courier is the only other character that can rival the sole survivor. He then gets defeated by a single shot to the head and almost dies were it not for a robot saving.
The sole survivor is trust into one very tough area where not even a team of Brotherhood could survive. He survives to find the man who killed his wife. Kellogg is a veteran of the wastelands and has been cybernetic enhanced. In order words he is like the courier. The sole survivor defeats him.
He has no level cap. He is healthier than everyone, And can be made to a ghoul. In one of his perks he can heal from radiation from the sun. He acquires a serum that makes almost immortal and able to heal from radiation.