In the vanilla game, Mr. House. That smug asshole insults even if you do work for him. Acting all high and mighty when, really, he's just some shriveled up old man in a mechanical casket. I end him every playthrough and enjoy doing it, damn it.
In a modded playthrough, I'd have to go with Marko, from Someguy2000's Bounties series. If anyone hasn't played it, I highly recommend playing the whole Someguy series. NV Bounties III was fantastic and I think Someguy did a great job with Marko. I wasn't sure whether to feel sad or angry during the final confrontation with him.
What is swag? Baby don't gank me, don't gank me, no more...