An Arch enemy in Fallout New Vegas?...

While not an enemy. Boxcars. The wounded powder ganger in Nipton. Both knees bashed in and a mouth like a lacquered trucker.
I shoot him literally everytime I make a new play-through. 
Never done his quest.

Ghost people in sierra madre...never thought i could actually develop such a hate against video game enemies.

Marko. No one else came close to the burning vendetta I had against that character

@IIHawkerII
Father Elijah, Willis, Caesar, salt on the wounds, -died in agony.

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.

Benny, first order of business is always send him to hell and take his Gun as a trophy.