There was an issue of X-Men, Uncanny X-Men Annual #11 I think, where some villain completely destroys Wolverine except for a drop of blood. This blood drips onto some power crystal which, matched with his healing factor, rendered him complete again.
Shit, Wolverine has...
Had his face shot off by Punisher,
Been run over by a steamroller in that same issue,
Been crucified by the Reavers,
Had his skin melted off multiple times,
Burned in the sun with Phoenix,
Had his adamantium ripped through open wounds and the pores of his skin in the infamous X-Men #25 by Magneto,
Been ripped in half in the Ultimate universe by Hulk,
Been beaten brutally by a number of people,
Successfully managed to unhatch and kill a Brood inside of his body before it infected him,
AND has managed to survive through all of it.
You say Xavier or Phoenix are the most powerful mutants?
SHEEEIIIT... Wolverine at least hasn't been killed or crippled.