The X-Men's most powerful teleporter, Magik, is in fact a Sorcerer Supreme. Illyana Rasputin is undoubtedly one of the most powerful mutants ever to walk the Earth. As one future version of Emma Frost observed, "We're talking about someone who at one time controlled Limbo like a god while also running around on Earth having a relatively normal superhero life. The scope of her power is... incalculable."
But just how powerful is the mutant who calls herself Magik (apparently because she genuinely doesn't know how to spell her mutant name in the English alphabet)? The last few years have seen her respected as a sorceress as well as a mutant, and she trained under Doctor Strange for a while; he even let her borrow the Eye of Agamotto on occasion. Magik's magical abilities are surely illustrated by the fact she's been recruited to teach at Doctor Strange's Strange Academy, Marvel's very own version of Hogwarts School of Witchcraft and Wizardry. Magik has long been one of the prime candidates to replace Doctor Strange as Earth's Sorcerer Supreme should he die in the line of duty, but while that's a huge honor, it shouldn't be a surprise, since Magik is already Sorcerer Supreme of a different realm.
The X of Swords Handbook confirms the scale of Magik's power - by revealing she is in fact already a Sorcerer Supreme. As the Handbook notes, "As the Sorcerer Supreme of Limbo, Magik's use of black and white magic is unparalleled there, and after additional training in the mystic arts her magic has become more powerful on Earth." This account of Magik's power fits perfectly with the comics themselves; during 2012's Avengers vs. X-Men event, Magik was able to defeat Doctor Strange while on her home turf, the Hell Dimension of Limbo, while Uncanny X-Men Annual vol. 3 #1 depicted a likely future in which she has taken his place.
Marvel Comics has naturally tended to focus on Earth as the be-all and end-all of magic, but in reality every single dimension is supposed to have a Sorcerer Supreme. On Earth, the line of Sorcerers Supreme began 1,000,000 years ago when the alien wizard Agamotto defended Earth as part of a team of proto-Avengers and left behind the Eye of Agamotto to choose his successor. As Doctor Strange has himself realized, to be Sorcerer Supreme of a dimension is to carry a tremendous weight upon your shoulders; you are sworn to protect your reality from powerful threats, while your rivals will circle desiring the prestige of defeating and replacing you. No Sorcerer Supreme ever gets to retire.
Comic book readers have long speculated just how powerful Magik really is. The X of Swords Handbook suggests she should be considered a member of a pretty prestigious club; she is the greatest sorcerer in an entire dimension. In addition to serving as a Captain on the X-Men's island nation of Krakoa - one of four elite guards charged with the safety of mutantkind - she is also sworn Sorcerer Supreme of Limbo.