(sry for bad spelling ahead of time)
Deadliest is like strongest, and it's a relative thing.
To answer this, you have to ask yourself who can kill wolverine? then who can kill apoloypse? Both are pretty good examples of who are difficult to take out throughout the series. There are others as well so you gotta think, who could kill everybody?
Nightcrawler can if he has a space suit (he can teleport the person into the sun but it'd probably kill him) Teleportation beats even psyhic to some degree, or at least can fight against it. The problem is that throughout the series most the teleporters were pretty limited. Nightcrawler is said to only be able to go two miles and that is a stretch.
Rogue is a good one as well as any energy absorber. Rogue had the weakness she had to touch you. I wish she would have dated another energy absorber as they should have negated each other out. (or did and I don't know about it)
Phoenix can eat a star. But Jean Grey can't... Jean Grey actually sucks pretty much. But Phoenix's power level is why she is so deadly, not her abilities.
Magneto actually could mess with people's minds through the iron in their blood and mess with energies through magnetism that weren't limited to metals (that's an explaination given to why he was able to block cyclops and others energy beams). Magneto really deserved in my opinion the ability to control gravity as well. He also shoulda worked with sinister for all I can tell. Overall not really deadly but he can beat both above.
I don't think anyone can beat everybody, that's really not a good thing to do in a comic or any story. Everyone needs a weakness.