The Siegebreaker beast was my favourite. You could spot him from the Keep in early Act 3 so you knew that fight was coming.
I particularly dislike where Blizzard's laziness poopoo'd on the possible immersion of this part. The siegebreaker picks up and eats the same guy infinitely there. Looks cool as long as you run past without stopping.
2 years in "polish mode" and they couldn't throw in an additional idle animation.