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Regarding the best demon lords...
I've gathered quite a collection.
Here he is again – Melkor Baugliir, the first and only.

Father of Dragons

King of the North

Lord of the Elements

Master of Diplomacy

Great Educator

Great Humorist

These aren't prisoners; they're raw materials.
Philosopher

The subject of worship of the first organized religion in Middle-earth

The archetype of all fantasy and a significant number of cartoon villains

P-progression.

Nenuacho.

A butterfly is essentially the same as a caterpillar. And a caterpillar is a pest, causing famine, plague, and, if you're lucky, riots, wars, and cannibalism. A true spawn of evil, far more so than orcs and dragons.
This is how his enemies see him.

And rightly so – let them fear.

Relatives, you see.

All evil comes from them. Or rather, all good comes from them...

This is almost canon, by the way. So, it's Varda (that's the same "Elbereth, Gilthoniel," if anyone doesn't know) who is to blame for everything. She chose the weakling and milquetoast Manwë, whom she then manipulated as she pleased – and Melkor went completely berserk after that.

Hey, where's Melkor?

Oh, never mind. Tell him hello when he gets back.
Although, you probably won't have time for that...