Like I've said before, Claremont obviously has plans for Rachel. There's way too much craziness going on with her for this to be her character permanently. And one can never go by who's beaten whom before. That only matters to a particular story. Galactus is pretty close to the top of the food chain, and he's gotten his ass handed to him several times. That's irrelevant anyway. You asked who is most powerful, not who always comes out on top. Not too long before that, Rachel created a localized black hole and closed it. Rachel can deconstruct any of the other women with but a thought. No one else comes close to that level of power. Regardless of battles, Rachel has the most power.
Yes, Jean is dead. Well, kinda.
Rachel did create it on her own. The other's helped her maintain control, but it was her power that created it. None of the others had power to create it, but they did have power to help her control and maintain it.
At her power level now? What is her power level now? Her current power is TK on a subatomic level, subconsciously at that. Even when she lost to Emma, she says that her power is off the charts. Let's also not forget that only omega TPs are able to access the Phoenix, which Rachel has. Her power really has no limit. You're referring to her state of mind, not her power. The amount and strength of her power isn't in doubt now. Her mental faculty is.
There are maybe 7-10 mutants (at most) whose power rivals Rachel's: one is her mother, two are her brothers, one is her future husband, and one is her future son.
I don't know that it's so much fluctuation as much as it is application and use, Fiery. Take Betsy: she could punch a hole through a brick wall, but couldn't pick up a dime on the same page. That's not a fluctation of power. And Marvel's stated that Rachel has the same power levels as Jean sans Phoenix, which means that she is an omega TP/TK.
The only ones officially listed are:
Phoenix
Iceman
Kid Omega
Elixir
I'm sure all of Jean's children are as well.