Grava 100%. He punishes misplays so hard if you are melee. The last time he killed me was when I got stuck in terrain while kiting him. That was absolutely frustrating.
In SR Janaxia can be annoying, too.
I actually kinda like the boss design of Grava/MQ 
But maybe I’m just a masochistic idiot that likes to get punished for making a stupid mistake every now and then… 
I recently had the pleasure of running into 2 Benn’Jars in Shattered Realm Immolation map with ground damage. What a PITA that was getting stun locked, taking ground damage and getting meleed by the other Benn over and over. I got through it but was surprised to see them in such close proximity to each other.
The encounter reminded me of this thread. I found a new hatred for Benn’Jar at that time.