Sukuna Remën.

Here's a quick and easy breakdown of Sukuna's techniques when he's in Megumi's body:

"Dismantle" is his basic slashing attack. Sukuna literally "slices" space: the strike penetrates defense and slices the target. It works almost automatically, very quickly.
"Cleave" isn't just a strike, but a technique that adapts to the enemy's strength: the stronger the opponent, the more powerful the strike. This allows one to penetrate even the most stubborn defenses.
Ten Shadows Technique (by Megumi) – Sukuna utilizes shikigami (summoned creatures) and their combinations to their fullest potential. He wields them far better and more dangerously than Megumi herself, and easily combines them with his cursed techniques.
Reverse Cursed Technique - allows him to heal and restore Megumi's body right in the middle of battle, so that serious wounds do not incapacitate him for long.
Mahoraga is the most powerful shikigami of the Ten Shadows, which Sukuna is able to effectively use and control. Mahoraga adapts to enemy attacks, and Sukuna uses this to disrupt any enemy strategy.

Features of power in this body
The main reason why Sukuna was so eager for this vessel was Megumi's Ten Shadows technique:

Sukuna gains access to all of Megumi's shikigami and technique combinations and uses them with much greater power and ingenuity.
He maximizes the technique's potential by combining it with his cursed techniques (such as "Cut" and "Split").
In this state, Sukuna is able to use the Reverse Cursed Technique for regeneration, literally "repairing" Megumi's body even after the most severe wounds.
Atmosphere of the image
The combination is especially eerie because of the contrast: Megumi's familiar features, combined with Sukuna's alien, ruthless essence. It's as if someone has donned a familiar face, but underneath it lies a completely different person.

What changes in behavior and presentation
Even if the body is Megumi, Sukuna moves and speaks in his own way:

Gait and posture. More relaxed, almost lazy, but it conveys immense strength and a readiness to instantly explode with aggression.
Speech. His tone is mocking and arrogant, often using words to mock his opponents and even Megumi himself.
Gestures. Sukuna likes to emphasize his power: slow, demonstrative movements, as if he's savoring the moment.

Appearance
Sukuna doesn't completely change his body—he uses Megumi's body as a vessel, but imposes his characteristic markings on it, causing the appearance to become a hybrid:

Black line tattoos. They appear on his face (around the eyes, forehead, cheeks), neck, arms, and torso—this is Sukuna's signature "pattern." The lines appear as sharp, angular marks, emphasizing the nature of his curse.
The look and facial features. The face remains similar to Megumi's, but the expression becomes cold, predatory, and visibly cruel. The gaze seems "heavier," lacking Megumi's softness.
Nails. Sukuna has long, pointed, dark blue or black nails—they immediately reveal his presence, even in someone else's body.
Additional details. In some scenes, Sukuna can temporarily change his face, opening additional eyes or mouths to heighten the horror effect and demonstrate his complete control over his body.
Clothes: Usually it's whatever Megumi was wearing (Tokyo Magic College uniform, etc.), but the tattoos and overall appearance make it look sinister.