The omega-level telepaths are: Xavier, Jean Grey, Emma Frost, Shadowking, Cassandra Nova, Quentin Quire and Psylocke.
Though I think it's a bit hard to differentiate 'phoenix Jean Grey' from non-phoenix Jean Grey just because she spent so much of her lifetime being powerboosted by a cosmic force.
But pretty much like others have said, it's up to the writer. t I feel like it's a common trope to try and say a telepath is one of the strongest on the planet to help prop them up. It's kind of like asking who is the strongest hero.. good luck. Their feats are unique to a specific situation (and often driven by an emotional or dramatic context) that it's not really easy or possible to measure such things.