It has to be vizier on heroic or lei shi + sha of fear. The MC coding is beyond horribly lazy and It's not exactly fun getting insta-killed by your 2 friends. Not to mention the the silly parts such as a mage being mc'd can end up spending 20 seconds just swapping armors back and fourth LOL or sometimes they just decide to take off and run away to the middle part of the whole room and just stand there and then come back if you let them be for awhile.
Lei shi is pure troll if you play a healer (i stopped screaming at people for putting dots on target's that are supposed to be CC'd, they just dont understand it even though I think i've died 5 times to that very reason) and it's overall a boring encounter.
Sha of fear: havent tried him on heroic but i assume it's pretty much the same thing there as on lei shi, overall just ends up being boring