Spoilers for PMMM...but the title itself is a spoiler so...
Akuma Homura is the ultimate evil of the Madoka-multiverse, originally a "normal" magical girl until she stole some of the power of the Goddess Madoka and ascended into her demonic form in order to protect Madoka and always keep her close to her.
But how strong is this devil anyway?
Physically speaking, Akuma Homura would be far physically stronger then any Witch should she physically manifest which means she would likely be far stronger then Kriemhild Gretchen, a witch so physically large that the Earth was only the size of one of her eyes, and was comprised of enough despair to end a universe.
Even though she has only a "fraction" of Madoka's power, Homura has the capacity to reality-warp an entire universe in seconds. Kami Madoka and Akuma Homura were also able to react to each other's magic briefly suggesting that their reactions are in par with this speed quadrillions of times lightspeed.
Homura is not quite an abstract entity although no longer purely physical either. Described as neither being quite witch or god, she is the Devil of the Madoka Multiverse and her barrier, the dimension each witch-being projects and warps as their personal domain, is large enough to contain the entire universe. For reference sake, Kriemhild Gretchen's was only stated to contain the Earth.
Homura has a wide variety of powers, even outside of her reality-warping. Even in her normal Magical Girl State she was able to stop time for unstated relative periods to her, and could reset timelines. She also has a magical shield which she can use in conjunction with spatial manipulation to essentially use hammerspace and pull anything she has stored in her shield out. During the Wrath Arc of the Manga, Homura the Magical Girl also gained telepathy and the ability to alter the memories of others.
In her witch Form, known as Homulilly, Homura gained the ability to protect her own barrier like all witches and to summon her familiars to serve her. During the Rebellion Movie she also demonstrated her ability to drain the power of others, even from conceptual entities like Madoka.
In her Devil Form, all her previous magical girl and witch powers are massively boosted. She's able to completely alter someone's memories at will or seal away all their powers. She was able to easily force the Incubators, a type IV civilization that act on the universal scale to serve her will by her sheer mental strength.
How would Homura do in other verses?
In the DC Universe, Homura would be equivalent to a Pre-COIE Level Powerhouse or a Post-COIE "Crisis" Level Event. In the Post-Crisis Universe, she would be a monstrous threat to the universe, though it would be possible for the Justice League to stop her if they all fought together. She would be approximately equal to a Pre-Crisis Strong League Member like Pre-Crisis Superman, Goddess of Truth Wonder Woman, and so on. Primarily her lack of speed might prove to be her undoing against people of this verse though her ability to seal off powers would make her very dangerous.
In the Marvel Universe, Homura would rank as a Cube Being generally speaking, similar to Post-Retcon Beyonder and Post-Retcon Molecule Man. She would be generally above Skyfather level beings like Odin in terms of raw power, though their speed, experience and versatility might allow them to fight semi-evenly against her. That said if she can steal the power from the concepts of Infinity, Eternity, Death, or Oblivion she would rise to be far more of a threat then her current position.
In the Final Fantasy Universe Akuma Homura would be roughly equivalent to a void being or the most powerful of sorceresses like Cloud of Darkness or Ultimecia's Final Form. She would be as strong as most of the highest tier but would still be below the ultimate top tiers of the verse, though her greater level of speed compared to most of the verse would give her a notable advantage.
In the Sailor Moon Universe, Homura's power to reality-warp the universe would put her above even the strongest Normal Sailor Crystals in the universe including the Silver Crystal, the Golden Crystal, the Saphir Crystal, and Black Crystal, however Homura's lack of experience and versatility compared to some Top Tiers in the verse make it possible that she would lose to some of the strongest characters in the verse like Queen Serenity or Sailor Galaxia. Against the strongest two characters of the verse it is highly unlikely she can defeat Chaos even in it's base form, although it is possible if she siphon's off it's power like she did Madoka.
In the Clamp verse, Homura would be able to easily have complete dominion over any individual universe's spirits and magicians she wants, and she could act as a local god or supreme spirit does similar to Mokona to Cephiro. She would likely be stronger then most of them given the versatility of her powers and her speed relative to them. If left alone, she could eventually overcome and control the multiverse however the supreme forces of the multiverse would be very likely capable of stopping her, most notably the high mages of Fei Wang Reed and Sakura Kinomoto.
In the Haruhi-verse, Homura would be capable of having complete dominion over entire timelines as Haruhi does, and would be able to overcome the advanced technology of the Data Integrated Sentient Entity similar to how Haruhi took control of the Incubators, which would give Homura essentially control of reality. That said Haruhi is unaware of her powers and if awakened would theoretically be far stronger then Homura similar to how if Madoka reawoke to her powers, she would be similarly.
In the Bastard!! universe, Homura's power would be above even that of Satan who is a gradual universal threat, and would be comparable to the power of God, the universe creator. That said the existence of Dispel Bounds and Eternal Atoms would likely slow down her reality-warp powers drastically and there are some techniques which may have the ability to kill her from Seraph-Level Entities and above, specifically ones which destroy on the physical, astral, and spiritual planes at the same time. It is unknown whether those would kill Homura since Homura is partially an abstraction in her Devil Form.
In Image Comics, Akuma Homura would be roughly comparable in power to God, Satan and Spawn's strongest forms and the four of them would enter a new half political half power game for control of the cosmos. That said she would still not be as strong as the Man of Miracles who is a high degree of multiversal.