Number 2: M'uru
This Naaru had a rather eventful history during a brief period of time. M'uru was the former guardian of Tempest Keep before Prince Kael'thas and his forces invaded and captured him. He was brought to Silvermoon where the Blood Knights began draining him for his energy. After Sunwell Plateau opened up, M'uru was again captured by Kael and brought to the instance and served as the fifth encounter. Part way through the encounter, he turns into a giant pseudo voidwalker-like being named Entropius.
Right, so on Eredar Twins, you had to bring about 8+ healers. On M'uru, you had to recompose the raid and bring no more than 6. I believe the formula for success was 4 Resto Shamans, 1 Holy Priest and a Resto Druid. The sheer amount of DPS required meant stretching out player capabilities as much as possible. If your guild didn't break up after wiping to the Eredar Twins repeatedly, then M'uru might've been the boss which caused them to throw in the towel.
Why it sucked
M'uru depended entirely on the skills of your DPS players. In order for them to do their jobs, it was up to the healers to ensure that they lived long enough. This wasn't a technical encounter or anything. It's a blitz fight that involved players DPSing M'uru and controlling or killing various mobs that run in or spawn.
Once M'uru falls, phase 2 began where he turns into Entropius. With luck, your raid group should have killed whatever mobs that were still alive (or at least have them low health). Now it's an "us or them" fight. Your raid is racing against Entropius. Either you're going to kill him or he's going to kill you. Your raid is going to take damage from Negative Energy. That ability gradually increased until your healers ran out of gas or were otherwise unable to keep up with the healing demands.
M'uru was some serious business.
What clocked in at number 1?
Number 1: Freya + 3 trees
Freya is a daughter of the titans. She primarily looked after the Conservatory of Life within Ulduar before being turned crazy by that Yogg-Saron fellow. Her hard mode is one of the toughest hard mode level encounters within Ulduar compared to the other keepers. Similar to Sartharion with 3 Drakes up, Freya's hard mode means leaving up her 3 tree attendants. Unlike Sarth however, they don't directly interact with the raid in the fight. Freya merely gains their buffs.
Without her tree elders up, Freya is fairly easy to take down. For the duration of phase 1, the fight centers around the raid working through 3 sets of trash mobs. Each set is taken out twice. The second phase is when Freya will actually take damage. Each trash mob you take down knocks out a certain amount of buffs from Freya. As the buffs are reduced to 0, Freya will eventually not gain any more health.
With the trees, it added extra layers of complexity. Elder Brightleaf increased magical damage by Freya and her allies by 60%. Freya would be able to drop Solar Flares on various targets (it's a big laser beam like thing). Stonebark increases her physical damage and she gains that really annoying Ground Tremor ability that deals physical damage and shuts down spell schools for 8 seconds if players are caught casting. Ironbranch ramps up the physical damage of her allies and Freya can cast Iron Roots on players that freeze them in place and applies a damage over time component.
With all 3 elements in place and Freya herself, the encounter becomes much more difficult for healers.
Why it sucked
On top of exploding and Detonating Lashers, micromanaging Stormlashers and Snaplashers as well as handling the big tree mobs, raid groups had to deal with an inane amount of extra crap. If healers aren't prepared, a well timed Solar Flare could wipe out a part of the raid. Every player has to be near 100%. Pesky Iron Roots are pesky. Sometimes a player gets caught out of position and wrapped up in Iron Roots. The roots would have to be targeted and destroyed.
Ground Tremors were especially annoying. It's fun casting a spell only to realize that you got caught in a Ground Tremor and are locked out from casting from your school for 8 seconds.
After patch 3.2, several nerfs were handed out. Freya's was no exception. For one thing, the Iron Roots can now be removed without having to simply DPS them. A PvP Medallion, or a Shaman with the Enhancement talent Earthen Power trivializes the fight greatly (by chain dropping Earthbind Totem).
From all accounts, I'd say that Freya with 3 Elders up is one of the hardest fights to heal in its class.
Everyone loves the honorable mentions list! This list contains some of your favourite fights and why they weren't included.
Algalon – This would've been an obvious candidate. Almost too obvious! The healers I spoke to about Algalon mentioned that once the raid got a feel for the encounter and how it was supposed to go, it would've been a piece of cake to reduplicate efforts.
Kael'Thas – The original last boss of Tempest Keep required very precise movement and healing assignments. But once your raid figured out each phase, Kael would be farmable from thereon out. It's the learning of Kael that was the hardest aspect. Once guilds got him down, they were able to sleepwalk their way through that fight. In contrast, Lady Vashj felt like a different encounter every time due to the randomness factor.
Archimonde – Ready for the ultimate "Get away from fire" fight? While this was challenging for healers, it was just as testing on every role. This was a simple evaluation of a player's ability to run and timing of their Tears. Focus here was on individual performance.
Sunwell Plateau – Any boss within this instance would have qualified. Kalecgos, Brutallus, Felmyst along with Kil'Jaeden could've been on the list here. No instance has ever been contained more challenging bosses than Sunwell. But alas, it wouldn't do to have all bosses here on the list.
Clearly there are going to be differences of opinion. How would you rank the encounters differently? Who would you replace and why?
Want some more advice for working with the healers in your guild? Raid Rx has you covered with all there is to know! Looking for less healer-centric raiding advice? Take a look at our raiding column Ready Check.