There was no villain in the series that could top Younger Toguro.
We see him first as the most powerful demon heading a mafia-like group involving other demons and corrupt humans.
He was working for Sakyo, whom wanted to open up a tunnel to Demon World.
He wanted to fight Yusuke Urameshi in the Dark Tournament, and before the finals were due, he absolutely destroyed a team single-handedly.
You find out he was originally a human, a very talented martial artist whose students were massacred by a demon.
He was once in love with Genkai, and they won the Dark Tournament together. They broke up because he wished to be a demon, and she couldn’t accept that at all.
And I was very sad when he killed her.
His muscles and sheer power were something else
His 100% strength form was absolutely terrifying.
I was glad he only pretended to kill Kuwabara.
But he managed to get Yusuke to unlock his true power.
And gave Toguro the biggest spirit gun ever.
And he died after almost stopping it.
Koenma offers him a very light punishment when he goes to the afterlife.
But he demanded to have the worst punishment in the afterlife, leaving a rather touching goodbye to Genkai.