Ulduar and to a lesser extent ICC hard modes were the best. I like when the jump in health and damage also brings a change of mechanics, like FL, freya, mimiron, vezax, yogg, deathwhisper, marrowgar, and nef heroic.
When it's just the same fight as normal except more damage, it looses the appeal and creativity what drew us to hard mode raiding. We like a challenge, not a "you have to do 10% more damage only to make the shorter enrage timer" on certain fights now. (particularly, rhyolith, staghelm, and baelroc)