To pick one the more difficult (but not the hardest) from Ulduar, the Assembly of Iron was a group of 3 dudes, and each time you killed one it would buff the remaining ones.
Assembly of Iron - WoWWiki - Your guide to the World of Warcraft
Hardmode was killing steelbreaker last, which means he's spamming a progressively heavier AoE, and when its down to just him he starts killing the tank every so often. He also gets partially healed and another buff to his damage every time someone dies when he's the last guy. The various abilities from the encounter required quick reactions and precise positioning, it really was a one person can fuck the whole raid encounter.