Some people say it is Xavier, other people say it is Jean and other that Emma would beat her.
There are also strong contenders like Exodus, Cassandra Nova or Sinister.

I want to leave X-man out of this thread because while powerful he hasn't been relevant in ages and i think he is still depowered (i maybe wrong with that). Cable and the Cuckoos are out too.

For this thread Jean doesn't have the phoenix but other characters can't count with any power ups or cerebro