For Yin its probably in the first episode of the Gaiden OVA when she uses her specter to force a contractor with the ability to control people to shoot himself in the head, saving Hei's ass.
Why are there so few entries here? For someone who kills with electricity and is repeatedly stated as a Badass, why are there so few mentions? I, for one, thought that Hei vs Luise in the first episode really made an impression of why he's called "The Black Reaper."
Hei taking out numerous contractors to get to Claude and Yin at the beginning of the 4th OVA episode is full of awesome.
Mina defeating a depowered Hei in the 11th episode of the second season.
Early on, when it's revealed that Hei's jacket is bulletproof and the ensuing fight sequence.
(after firing several shots into Hei's back)
Alan: I can't believe Luise got himself killed by this guy, I really thought he'd put up more of a fight.
Mao: He will. And when he does, you might want to keep in mind: Hei doesn't wear that coat as a fashion statement.
Mao: It's bulletproof.
The amazing teamwork between the members of the Secret Intelligence Service, specifically between November 11 and April.
Speaking of November 11, his introduction and subsequent fights throughout the series are all awesome but it's his killing Decade and a room full of heavily armed agents using nothing but a bottle of bourbon that really takes the cake. Sure it happens off screen, but still.