Horus Heresy - Fuck, that's a hard one. Like many people it seems, I liked a lot of the human characters in Legion. Though I gotta say my favourite there is Grammaticus. He's like an uber spy and master manipulator who's been around for ages. Plus, he took out a Lucifer Black unarmed. That's some old school badassery right there. Also, his end, after he realised the large scale effects of his actions, was quite memorable.
Modern - This is a bit of a random one but I'd probably go with Toros Revoke from Ravenor Returned. That guy took kicking ass to a whole new level. Fighting the Brass Thief in melee was quite epic, seeing as pretty much nearly everyone else who went toe-to-toe with it ended up sliced to ribbons. Not to mention he would have taken Ravenor in that psychic duel if Nayl hadn't interfered. An interesting character, he should have been featured more in my opinion.
The human appendix. Proof of a higher power. A divine kill switch so to speak.
No one really likes a smartass, but people tend to like a dumbass even less.