It has to be Castlevania Lord of Shadow's Titan boss fight in Sanctuary of Titans - chapter 2.
I'd say Skorge battle in Gears of War 2 is second worst.
Overall, I think QTEs are the worst thing that happened to gaming. Especially shitty ones that don't give you any kind of feedback if you do something wrong: you try doing them faster - you fail, you try doing them slower - you fail, you try pushing the button earlier - you fail, you try pushing the button later - you fail.
I've just wasted almost one hour for the final sequence in Castlevania in which you finish off this rock Titan, I know that I used to spend 2 hours on Skorge battle in Gears of War 2, and it was probably even more annoying, but still this shitty Titan battle takes the cake.
QTEs should die - someone should simply finish them off, nuke from the orbit, but without any button prompts.