[Bloody Sam Roberts]

For me it was Klingon Commander Kruge. in ST4: Search for Spock. Heartless, ruthless and he was killed off, very satisfyingly, in his only appearance.

I love it when the bad guys get it in the end.

Kruge: A failure? The most destructive force ever created? You will tell me the secret of the Genesis torpedo.
Saavik: I have no knowledge.
Kruge: Then I hope pain is something you enjoy.

.

David Marcus: I'm sorry, sir. Just don't surrender. Genesis doesn't work. I can't believe they'd kill us for it.
Kruge: Admiral, your young friend is mistaken. I meant what I said. And now to show that my intentions are sincere, I shall kill one of the prisoners.
Kirk: Wait a minute! Give me a chance to talk.
Kruge: [in Klingon] Kill one of them. I don't care which.

[COBOL2Java]

"To the last, I grapple with thee; From Hell's heart, I stab at thee; For hate's sake, I spit my last breath at thee."

[BenLurkin]

Evil Doc Brown.