Final Verdict

Raw Power: Chainsaw Man (by far)

Speed/Strategy/Techniques: Itachi

Win Conditions:

Chainsaw Man wins through overwhelming physical dominance and erasure.

Itachi wins through sealing or mental incapacitation — extremely difficult, but canonically consistent with how Full Chainsaw Man has lost before (via conceptual or supernatural sealing/defeat).

So in short:

If Chainsaw Man can be beaten, Itachi’s methods — sealing the soul and attacking the mind — are exactly the kind that could do it.