Knives x Meryl. My second favorite pairing in the Trigun fandom. (So much so that I co-run a fanlisting for the two, located here.) Why? It's definitely not for the redemption factor, at least for me. I think it'd take a hell of a lot more to redeem Knives than some good humping, though it certainly wouldn't hurt. Mostly because it's so evil and wrong. He's evil and hates her kind, she's got Big Feelings for Vash--OMG THEY'RE PERFECT FOR EACH OTHER!
In the fic world that exists in my head, the ideal KxM set-up has Knives being all Machiavellian and sexy and them using each other. Sadly, I've only ever seen one doujinshi of this pairing--how wrong is that if Midvalley x Band and Wolfwood x Meryl have multiple books? Wonky.