You must be joking (assuming you're talking about Geyser). It has to rank as one of the most horrible mechanics ever designed. In one of the hardest bosses in the history of this game, this one was so bad that people had to cheese it in order to make it manageable. As a stand alone mechanic it might not have been so bad, but mixed with everything else in that phase 4 it's easy to see why Ragnaros destroyed so many guilds.
As for my personal favourite, it's the additional drakes in the Sartharion encounter. A mechanic that was organic, didn't need a toggle, was a pre-cursor to Ulduar's fabulous hard modes, and could be staggered in difficulty while you learned it.