When Iceman was introduced in X-Men #1 (by Stan Lee and Jack Kirby), he was a fairly straightforward mutant. Bobby Drake was originally able to encase his body in a snow-like shell, and manipulate ice and cold, However, over the decades, Iceman's powers and potential grew, eventually revealing him to belong to an entirely different class of mutant: Omega-Level
As an Omega-Level Mutant, Iceman is one of Marvel's most powerful beings. He's discovered greater control over his powers throughout the years and Bobby has proved exactly why he earned his distinction from the rest of the mutant population.
When Did the X-Men's Iceman Discover His Omega-Level Powers?
Iceman was a founding member of the X-Men. As a mutant with elemental powers, he has always been able to generate a field of subzero temperatures around him, cooling and condensing any moisture in the nearby atmosphere into ice. At first, he typically used this power in fairly pedestrian ways, creating ice constructs and slides.
He eventually discovered how to effectively cover his body in an ice-like shell, granting him more durability and control over his abilities. Over the years, his true potential became more apparent through the help of others. Scott Lobdell and Tom Raney's Uncanny X-Men #291 and #292 saw Colossus' brother Mikhail Rasputin use his powers to transform Iceman's body into his "final form," revealing that Drake always had the potential to completely transform into a body of organic ice instead of merely using it as an outer shell.
Iceman continued to experiment with his powers, particularly after Emma Frost took control of him in Uncanny X-Men #314 (by Lobdell and Lee Weeks) and revealed that he can exist without a body. Iceman proved able to exist as a sentient presence by controlling his very molecules even when his body was destroyed. He was then able to use the surrounding moisture in the atmosphere to create a new body. While controlling him, Emma was genuinely shocked at his potential and later taught him how to better control his abilities in Uncanny X-Men #331 (by Lobdell and Bryan Hitch).
Since then, Iceman has become even more powerful, proving capable of creating legions of ice copies. This allowed him to impressively form an entire army out of himself in Wolverine & the X-Men #2 (by Jason Aaron and Chris Bachalo).
How Does the X-Men's Iceman Compare to Other Omega-Level Mutants?
Variants of Iceman also proved to be Omega-Level mutants, most notably his Age of Apocalypse incarnation, who was perpetually in his ice form and could turn molecules into ice that he then transported alongside himself. As a result, this version of Drake was virtually indestructible, with attempts to destroy his body only temporarily slowing him down. Iceman's innate control over temperatures has also increased to the point where he can effectively remove the cold from areas, thereby actually making the temperature rise. In essence, Iceman is a living weather pattern, possessing a more specialized version of the X-Men's Storm's control over the atmosphere.
Iceman has reached such a point of control over his abilities that he can now easily be classified as an Omega-Level mutant. His powers appear to have no external limit, and have yet to be matched by any other living being. During the Hellfire Gala, Iceman was one of the Omega-Level mutants recruited by Magneto to terraform Mars, and almost effortlessly created entire polar ice caps, effectively bringing water back to the planet and helping life to thrive on the newly dubbed Planet Arakko. It sounds like an incredible feat for the deceptively powerful Iceman, but in actuality it's just another day on the job.