Demogorgon wins because i, the DM, say so However, by the 5e stat block demogorgon is very underpowered. They had a chance to correct this in MToF, but they went the otherway for some reason. Personally, I default to the 1e statblock when I rank relative power of fiends and then go from their.
So we have: Demogorgon - Asmodeus - Orucs (a rather distant 3rd)
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Notice Orcus has 40% fewer HP and a 25% weaker AC and less magic resistance too. I've revised my 5e Demogorgon accordingly
Since that edition Asmodeus has been promoted to Greater God, if Asmodeus comes directly for Demigorgon and Orcus together, they run the other way at this point. Luckily for them its not his style.
As for Tiamat of course she wipes the floor with them too, she's a full god, even if her stats suggests she's a 2 dimesional monster instead of a God.
For perpective Orcus isn't that much higher in CR then an Empyreans, what like only 3 CR higher? the most powerful none deity Celestial (displacing Solars from that position).
Actually an Empyrean could probably take Demogorgon out, they have an incredible flight speed, huge range for their bolts (which can be any energy type except poison, necrotic, and maybe pyschic), plus earthquake and Fire Storm.
And for defense Legendary Resistance and Magic resistance. Oh and Dispel Good and Evil.
Even if the Empyrean dies their Divine Parent just brings them back to life, to hunt Demogorgon down again.
Still people forget that CR is in relation to PCs, even designed based on monster vs. monster fights.