If we were to rank the most brutal instance that ever appeared in the game, I would argue that week one Heroic Stonecore is up there: there were mechanics that would one shot you if you fucked up, specifically on the worm boss and Ozruk. Blizzard had to nerf that instance (as well as Heroics in general) at least a week into the expansion due to the difficulty. The second highest for me would either be Shadow Labs or MgT (especially when it first opened), Shattered Halls was hard but it definitely wasn't the hardest in that expansion.