I'd say Stonecore, Grim Batol is a definate second, but the bosses there are only hard because they require Dps to burn down adds before they reach a certain point, this can be a problem for noob Dps, but as with even current level gear these adds are starting to go down much faster I can see a few tiers time these bosses becoming MUCH easier.
Stonecore however is hard because of all the insta-gib mechanics. take the typical Dps, give him tier retarded.. Corborus? yup I can do 50k dps, shame I don't get to because I die on the first Submerge. Slabhide, easy boss, unless you get pwnt by the Crystal storm... Ozruk? do I really need to explain? three chained moves, each one possible to onshot... Sith Lord Azil, but if you've made it through the other bosses, you'll be fine on this one.
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For Pugs it's possibly Grim Batol, but in pugs as well as even guildgroups with non-default tanks (dpsers in OS) Ozruk in Stonecore is a killer.
Surprised at all the Stonecore votes, especially the Ozruk QQ....avoiding the huge 10 second cast time ground slam (or w/e it's called) cast is hard? Putting a dot on yourself while the boss has a reflective shield up is hard? Or is whacking Ozruk once, then running away from him in a straight line after an obvious stun effect the hard part? Yikes.
Sure, there are lots of insta gib mechanics, but that doesn't mean they're hard. If you can't avoid the gigantic rock coming towards the obvious dust effect on the ground, or can't run behind the boss casting a one-shot frontal cone attack in time, then you need to learn how to play better.
clearly Grim Batol...it's doable but I'd rather won't PUG it
voted lost city because of the achievement on Siamat, although I honestly can't imagine anyone having trouble with any of the non-achievements.
Nothing compares to Throne of the Tides when it comes to bad groups, the trash will own you with chain lightnings.
Also, on first boss, the avoiding of whirling things, interrupting and not staying in the pools is just too much for some pugs.
The same goes for 3rd boss in Shadowfang Keep, interrupting and CC seems just too much to ask for...
On the opposite, I never had problems in Grim Batol.
meh they've all been pretty easy since release but voted throne of tides for some odd reason
I have found Grim Batol overally hardest. All bosses are pretty painful with random groups.
Halls of Origination has the first boss which used to be the group destroyer at the beginning of cata.
Other hard bosses are Ozruk and sometimes Corborus in Stonecore.
Then for trash: Hardest one seems to be Vortex Pinnacle.
Fair point Krothar, I dont see many shamans about so I'm used ot people cleansing poison left right and center! :P
Most heroics are only hard if the PUGs are noobs. Which unfortunately in the case of DPS happens a lot. I personally see bosses in Grim Batol being fairly easy, I did Throngus on first try and Deathbringer isn't too hard but DPS ALWAYS miss the fire adds and don't move quickly when the dragon does his breath.
I don't think I can look forward enough to our friend being geared enough for heroics so we can take him along and complete our 5 man guild group of doom. Even taking one PUG member along worries me particularly if he doesnt understand CC or is a Death Knight.
After finally hitting 85 on my druid last night and heading into regular 85 instances, followed by a heroic on my paladin, I change my vote. Grim Batol has got to be the stupidest instance ever. Three times, my druid was placed "randomly" into Grim Batol in various stages of completion. Still haven't killed the final boss with that alt. Then, on heroic on my paladin, my paladin was placed into Grim Batol. I was like "What the...?" I think Grim Batol has become the Halls of Stone of this expansion, and the dungeon finder likes it far more than it should.
Grim Batol is the hardest mechanic-wise by far, Throngus was absolutly brutal during the first couble of weeks and even now if you get bad luck and get shield when there is a bunch of rocks and fire from mace phase you can still wipe even if its a guild run
It all depends on the players. Personally I voted Deadmines, because it is the hardest if you are lacking gear. I remember the first couple of days of heroics - I was able to kill just about any boss in any heroic with my guild in 2-3 tries max till we figured out the strategy. Until one night we got in the deadmines and wiped for 3 hours straight on Admiral Ripsnarl. Of course , heroics seem to be much easier with current gear, but Ripsnarl has a special place deep in my heart since that night!
Another fight that needs fixing - Commander Springvale in SFK - encounter is way too hard.
How refreshing. Honestly thought I would get a load of grief back from that. It appears some peopls can be polite
I would love to see it done, as I just can't imagine doing it myself. Although I don't have castable heals (DK heals don't really count IMO), so who knows. Might try it though :S Let me know via PM if you can.