Ryomen Sukuna vs Muzan Kibutsuji

Conclusion: Ryomen Sukuna wins with fairly high probability.
If we set the conditions normally as "both at full power, with abilities, fight to the death," it's about Sukuna 9:1 Muzan.

The biggest reason is that the scale of attack power and combat speed is too different. Muzan is overwhelming in the world of Demon Slayer, but fundamentally he is a close- to mid-range type built around tentacles, Blood Demon Arts, poison, and super regeneration. Sukuna, meanwhile, can fire off massive numbers of slashes from long range, and he even has Domain Expansion.

What is especially harsh for Muzan is Malevolent Shrine. Inside the domain, "Cleave" and "Dismantle" are continuously rained down over a wide area, so it interacts extremely badly with Muzan's strength of "regenerating even when cut." The moment he regenerates, he is chopped to pieces again.

However, Muzan has one big problem.

Sukuna cannot produce sunlight.

If we strictly apply the rules of the Demon Slayer world and set it as

"Muzan absolutely cannot die except by sunlight or special conditions,"

then there is a possibility that "Sukuna dominates in combat, but cannot completely kill him."

That said, Sukuna also has flame attacks. As far as the firepower he showed in the story goes, the key point is whether he can incinerate Muzan to the level where not even a regenerable piece of flesh remains. Since it is hard to believe a demon can regenerate if completely erased at the cellular level even by something other than a Nichirin Blade, I think Sukuna can ultimately finish him off.

A comparison looks like this.

Ability	Sukuna	Muzan
Pure attack power	Crushing victory	△
Long-range attacks	Crushing victory	○
Combat skill	Crushing victory	△
Regeneration	○	Crushing victory
Poison and biological attacks	○	Extremely strong
Area attacks	Crushing victory with Domain Expansion	○
Instant-death-level attacks	Extremely many	○
Toughness	◎	Abnormal

Furthermore, unlike Muzan, Sukuna has an extremely high battle IQ.

While Muzan has overwhelming physical ability, his fighting itself is fairly brute-force. Even in the final battle, his basic tactic was swinging around huge numbers of tentacles at high speed. Sukuna, on the other hand, observes the opponent's cursed technique, analyzes how it works, and changes tactics on the spot.

So when they face off, it would probably go like this.

Right after the start → Muzan deploys a huge number of tentacles
→ Sukuna casually dodges and cuts through them

Muzan "Why won't this guy die?"

No, rather than that, it's the reverse,

Sukuna "You're quite persistent."

and he confirms the regeneration ability.

↓

Sukuna increases the density of his slashes.

↓

Muzan tries to turn into pieces of flesh and escape.

↓

Malevolent Shrine.

↓

He and his flesh pieces are continuously sliced on a scale of several hundred meters in radius.

↓

If he still regenerates, incinerate him with flames.

That would be the flow.

And in terms of personality, it's quite interesting too.

When Muzan encounters an opponent clearly stronger than himself, there is a very high chance he chooses to flee because of his trauma from Yoriichi. Sukuna, by contrast, is the type who enjoys an opponent all the more the closer they are to his equal.

Because of that, from partway through,

Muzan: "This one is the same as Yoriichi. I must run."

Sukuna: "You're running?"

there is a high chance it would develop like that.

However, if it becomes "Sukuna vs Yoriichi," the discussion gets a little more interesting

More than Muzan, Yoriichi Tsugikuni is far more interesting as a matchup for Sukuna.

Whereas Muzan tends to be an existence who "can't be killed but is persistent" against Sukuna, Yoriichi's pure swordsmanship, speed, and insight are abnormal, so

Sukuna vs Yoriichi

would become a debate over "even if Sukuna wins, how far can Yoriichi push him?"

In Muzan's case, it is quite clearly

Sukuna ＞＞＞ Muzan.