As one of Marvel's premier superhero teams, the X-Men have faced many dangerous threats over the years. Some of their most deadly enemies are classified as "Omega-Level" mutants who have reached the upper limits of their specific powers, such as Vulcan, Proteus, Exodus, and Magneto, the Master of Magnetism. There is also the quietly powerful Mister Sinister, a mutant who has established himself as one of the X-Men's greatest villains, although he has the distinction of not debuting as a mutant. Nevertheless, Sinister stands as one of the most twisted antagonists the X-Men have ever faced, and has the potential to become the most powerful mutant in the Marvel Universe.
Nathaniel Essex, the man who would become Mister Sinister, was introduced to the world of the X-Men through one of their greatest tragedies. Sinister ordered the slaughter of a group of mutants known as the Morlocks at the hands of the original Marauders in the "Mutant Massacre" crossover that took place primarily in Uncanny X-Men #210-213 (by Chris Claremont and John Romita Jr.). The X-Men heroically rushed to defend their fellow mutants, and the resulting battle saw many members of the team seriously injured. Although the massacre served as the villain's first appearance (albeit in silhouette until issue #221), it was later revealed that he has a history with founding X-Men Cyclops and Jean Grey dating back to the pair's early childhood. It has also been revealed that Sinister's primary powers of agelessness and telepathy were granted to him by the mutant Apocalypse, another major adversary of the mutants, in the 19th century. These abilities, combined with Sinister's endless supply of clones has made him virtually immortal.
The abilities granted to Sinister by Apocalypse come in addition to the mutant abilities the genetics genius has granted to himself. Essex wasn't born with an X-gene but coveted the power of the gene and sought to create a mutant clone of himself. Upon his eventual success, Essex gained the formidable ability to control every molecule of his body. He also notably used his mastery of genetics to assign himself the abilities of the first fallen member of the X-Men, Thunderbird. Additionally, 2019's Powers Of X series (by Jonathan Hickman and R. B. Silva) established Sinister as a viable threat to the future of both mutants and humanity.
In a future timeline, Mister Sinister is given free reign by the Krakoan council for his genetic breeding experiments. This leads him to create Chimeras, clone/mutant hybrids combining the power sets of up to five mutants. He also betrays Krakoa, and creates a Chimera hivemind that annihilates the mutant society along with the majority of its inhabitants. It's a terrifying end that services few but Sinister himself.
This future gives a clue as to why Mister Sinister is potentially the most powerful and dangerous mutant alive. After relentlessly gathering the genetic material of nearly every mutant in the Marvel Universe during House of X, Sinister now possesses the means to bestow upon himself nearly any combination of powers and abilities. Complicating matters is the fact that Sinister serves no one's interests besides his own, which are often contrary to what is best for mutant society. Worst of all is the fact that the machinations of Mister Sinister is one of the primary reasons why mutants are destined to lose in the end according to Moira MacTaggert.
At the moment, Mister Sinister is still allied with Krakoa, at least on the surface. Despite his shady dealings behind the scenes and self-serving attitude, Sinister is recognized by Professor Xavier as a necessary evil to the continuous running of mutant society. Without Sinister's knowledge of genetics and cloning, the mutant resurrections that have become a vital part of life on Krakoa could not exist. Therefore, Sinister is tolerated, his devious plots are ignored, and he has even been granted membership in the governing body of mutant society, the Quiet Council. As it currently stands, none of the mutants seem to be able to see Sinister as the deadly threat he really is.