In X-Men stories, mutants can see a fair amount of variation when it comes to their powers. Sometimes, a mutant’s power is useless. Other times, well, it wouldn’t be too much of an exaggeration to compare the so-called Omega-level mutants to gods and goddesses. In fact, there are some Omega-level mutants who are so powerful that they might even be capable of challenging a character like Thanos who operates on a much, much bigger scale than most Marvel Comics characters.
Franklin Richards is the son of Reed Richards and Susan Storm. His situation is rather strange because neither one of his parents are mutants, but Franklin has a strong claim to being the single strongest mutant that can be found in the entire Marvel Comics setting. This is because he is a reality warper powerful enough that his subconscious can create a pocket universe capable of containing a replica of Earth-616. Something that has enabled him to capture the interest of Galactus and other cosmic entities in spite of the fact that he is still a child. As such, it seems reasonable to say that a fully-grown Franklin would be one of the best mutants for taking on someone like Thanos.
On her own, Jean Grey isn’t capable of matching Thanos. She is a very powerful telepath as well as a very powerful telekinetic, but she is far from being capable of fighting someone on Thanos’s level. However, the fun thing about Jean Grey is that she is one of the most compatible hosts of the Phoenix Force, which can provide her with a wide range of powers beyond what she already possesses. With them, Jean Grey has proven herself to be capable of taking on Galactus, who tends to be stronger than Thanos under normal circumstances.
Legion is the son of Charles Xavier, so it is perhaps unsurprising to learn that he possesses incredible power. Unfortunately, Legion tends to be hindered by the fact that his mind is home to more than a thousand personalities. In some cases, Legion absorbed these personalities into himself from other people; in other cases, they have sprung out of his own mind. Regardless, each personality possesses its own particular superpower, meaning that Legion is one of the most versatile characters that can be found in superhero comics. However, it is clear that Legion isn’t lacking in pure power, as shown by the fact that the “true Legion” personality could warp reality to wipe Elder Gods from existence.
Members of the Summers family are often ridiculously powerful. This happens to be true for Hope Summers even though she is the adopted rather than biological daughter of Cable, who in turn, is the son of Jean Grey and Scott Summers. Regardless, Hope is a host of the Phoenix Force. Moreover, Hope’s power enables her to mimic the powers of other mutants but at a higher level of power than what they can manage. Combined, that should be enough to enable Hope to take on Thanos, assuming that she had the right mutants situated in close proximity to her anyways.
Speaking of which, Cable is actually a pretty good candidate for a mutant who can take on Thanos. This can sound strange because the character is best-known for running around with various guns, but Cable is supposed to be a very powerful telepath and telekinetic in his own right. Unfortunately, much of Cable’s superpowers are tied up with keeping the Techno-Organic Virus under control, meaning that he is essentially operating under a serious handicap in most situations. Should Cable be cured of the Techno-Organic Virus, which is something that has happened in the comic books, he might be capable of providing a challenge to Thanos with some time to adjust. After all, whenever Cable has been cured of his condition, his powers tend to shoot up until they reach the same approximate level as those of Cable’s evil clone as well as Cable’s alternate timeline half-sibling Nate Grey, both of whom are very powerful even by the standards of Om