Personally, I'd say either the Patriarch or Abomination. The latter can be incredibly frustrating to fight due to its mines and bile monsters. You can quickly get overwhelmed, not to mention that he got a pretty nasty damage buff lately (that was somewhat toned down from what I've gathered). Meanwhile, the former usually kills me at his very last stage, when he gets the ability to shoot while running.
On the other hand, I think I only got killed ONCE by the King Fleshpound.